* Q1 net income 51 cents per share beats estimates

* Revenue falls 7 percent

* Cost controls, tax benefit help offset decline (Adds cost control details, tax benefit)

BOSTON, April 30 (Reuters) - U.S. research firm Morningstar Inc (MORN.O: Quote, Profile, Research) reported a better-than-expected 8 percent rise in first-quarter net income as cost controls and a tax benefit helped offset a revenue decline.

Morningstar, best known for its system of rating mutual fund performance, earned $25 million in the first three months of the year, or 51 cents per share -- beating the 39 cents expected on average by analysts surveyed by Reuters Estimates.

That compared with $23.1 million, or 47 cents per share, in the year-earlier period. Revenue in the quarter fell 7 percent to $116.7 million, which the Chicago firm attributed mainly to weakened sales to investment management companies.

Net income held up because of cost controls that kept operating income flat at $34.6 million from $34.7 million a year earlier, chief executive Joe Mansueto said in an interview.

Chief Financial Officer Scott Cooley also noted a one-time tax allowance in the first quarter. He said the company doesn't give guidance on future earnings.

Morningstar shares were little changed in after-hours trade from their Nasdaq closed at $39.67, up 1.1 percent. The stock is well-off its 52-week high of $76.95 reached May 7. (Reporting by Ross Kerber. Editing by Jason Szep, Leslie Gevirtz)