SAN FRANCISCO — Yahoo Inc. has retooled its online search engine to make it more helpful and engaging, joining an industrywide wave of improvements that so far haven't dented Google Inc.'s dominance.

The Sunnyvale-based company regards the upgrade, to be announced today, as the most significant change to its search engine since it reclaimed control of the underlying technology nearly 3 1/2 years ago.

If nothing else, the improved search engine should boost Yahoo's employee morale because it backs up co-founder Jerry Yang's vow to re-establish the company's reputation as an Internet innovator, said Standard & Poor's equity analyst Scott Kessler.

Yang returned to the company and replaced Yahoo Chairman Terry Semel as chief executive in June.

"Is this (upgrade) going to be very important in terms of market share? I'm not so sure. But it probably will help them in terms of mind share," Kessler said. "One of the reasons that Google has captured the imaginations of so many people is they have been doing so many different and interesting things."

Between June 2000 and May 2004, Yahoo relied on Google to power its search engine — a decision that helped transform its once-smaller rival into a juggernaut that now fields more than half of the search requests on the Web.

As part of its makeover, Yahoo will suggest ways to phrase a search request as a user types into the query box, it will provide a list of related concepts and it will produce more links to photos, videos and music on the main results page.

The search engine also will pull information from Yahoo's calendar service, Upcoming.org, to highlight local events when they're relevant to a request.

Many of Yahoo's new features are similar to improvements that Google and www.Ask.com introduced during the spring and Microsoft Corp. matched just last week.

The lag makes it seem as if Yahoo is simply trying to catch up to the rest of the pack instead of taking the next technological leap ahead in search, said Jim Lanzone, who runs www.Ask.com for InterActiveCorp.

"We are already looking ahead to our next improvements," Lanzone said. "Search is such an unsolved problem that it requires constant innovation to satisfy users. And the more search delivers, the more the users expect."