By Bill Rigby

SEATTLE (Reuters) - Oracle Corp has a habit of beating Wall Street expectations, but some investors fear billionaire Larry Ellison's software and server juggernaut will find it hard to keep up its own pace when it reports earnings and sets new forecasts on Thursday.

The world's No. 1 database software maker is facing wobbly economies across the world, especially in Europe, and might already have reaped the greatest gains from the purchase of Sun Microsystems 18 months ago.

"It ain't easy to keep putting up those kinds of numbers," said Richard Williams at Cross Research, which has no holdings in Oracle.

"The real question's going to be: Do they maintain the kind of growth rates they have been seeing? Or do we a see a slower growth rate implicit in first quarter guidance?"

Oracle, based in Silicon Valley's Redwood Shores, has almost doubled sales and profit in the past five years, helped by a string of acquisitions. It has beaten Wall Street's average sales and earnings estimates for the last six quarters -- often by a good margin -- and topped annual estimates for the last five years.

Aside from that stellar performance, technology investors pay attention to Oracle's quarterly earnings as its fiscal quarters are out of sync with most others and it is the first to give a glimpse of business conditions in the most recent months, in this case April and May.

For Oracle's fiscal fourth quarter, ended May 31, analysts are expecting a 12 percent jump in sales, to about $10.75 billion, and an 18 percent leap in profit, to 71 cents per share. Several have hinted they expect those estimates to be beaten.

But most are focused on the company's forecast of the current quarter's new software sales -- delivered in a conference call after the earnings report by Chief Financial Officer Safra Catz -- which are key to long-term growth.

"I wouldn't be surprised if they forecast a little bit of disappointment going forward, talking down the analysts' estimates for revenue," said Kim Caughey Forrest, senior analyst at money manager Fort Pitt Capital Group, which holds Oracle shares.

Investors may already have priced in some disappointment. Oracle's shares are down 10 percent since the end of April, compared with a 7 percent fall in the tech-heavy Nasdaq. It still trades at about 13.5 times expected earnings for the next 12 months, compared with an average of about 10.7 for its peers.

WEAKNESS, DEALS AHEAD?

Oracle, which vies with Germany's SAP AG and IBM Corp in selling specialized software to companies, and with Hewlett-Packard Co in server hardware, has been largely immune to persistently weak consumer sentiment since the U.S. recession ended two years ago.

But a recent loss of momentum in the U.S. economic recovery and renewed fiscal woes in Greece put a question mark over continuing demand.

Oracle's fiscal fourth quarter is usually its strongest, as it signs deals before its financial year end, but the current quarter is generally its weakest. Investors will have to judge how much of that weakness is more than cyclical.

"In the last four quarters, Oracle has managed to be independent of the rest of the group," said Williams. "What we are looking for in the quarter is to what degree the first quarter will be seasonally weak."

Software maker Adobe Systems Inc warned that lower-than-expected sales in Europe dragged on earnings this week, sending its stock down 6.5 percent on Wednesday.

In the face of faltering economies, Oracle, which has spent more than $42 billion on acquisitions over the past six-and-a-half years might be ready to go back into the market to help keep up its growth rate.

Eighteen months after Oracle's $5.6 billion deal to buy hardware maker Sun, which strongly boosted sales and provided a new set of clients to sell software to, the year-over-year comparisons are getting harder to match.

"They haven't done a large acquisition in a while, that's why I'm assuming we're going to see a little leveling off in year-over-year gains going forward," said Forrest at Fort Pitt.

Some potential targets mentioned by industry watchers include software companies Symantec Corp, BMC Software Inc and CA Techologies, formerly Computer Associates Inc.

"We're getting closer to the point where they are going to have to do another deal," said Williams at Cross Research.

(Reporting by Bill Rigby; editing by Andre Grenon)