* Private equity heavyweights Apollo, Blackstone report Q4 results
* Rising U.S. stock market help buyout firms book gains
* Blackstone assets under management hit record high
NEW YORK, Feb 1 (Reuters) - Alternative asset managers Apollo Global Management LLC and Blackstone Group LP reported better-than-expected quarterly earnings on Thursday, as a U.S. stock market rally buoyed the value of their private equity holdings.
Investors poured a record amount of cash into private equity last year, boosting asset-based fee revenues. Investment gains through the end of 2017 and the U.S. tax overhaul signed into law in December suggested a healthy pipeline of carried interest fees in the next few years.
A 6.1 percent rise in the S&P 500 in the final three months of 2017, the index's biggest quarterly rise in two years, boosted company valuations and allowed buyout firms to book hefty gains on some of their portfolio holdings.
Apollo's private equity funds appreciated 9.1 percent in the fourth quarter, helping drive economic net income per share to$1.22, well ahead of analysts' expectations of 66 cents, according to Thomson Reuters I/B/E/S.
The value of Blackstone's private equity funds, which account for around a quarter of its assets, rose 6.8 percent in the quarter. Its economic net income came in at 71 cents per share, ahead of estimates for 67 cents per share.
Economic net income reflects the mark-to-market valuation gains or losses on a firm's portfolio and is a key earnings metric for U.S. private equity firms.
The private equity industry is also coming off of a bumper year for fundraising, with the industry attracting a record $453 billion from investors in 2017.
Blackstone, the world's largest manager of alternative assets such as private equity and real estate, said its assets under management rose to a record $434.1 billion in the fourth quarter amid a flurry of fundraising.
The healthy investor capital inflows are a boon to fee-related earnings that are linked to a management fee on the assets held for investors in the order of 1 percent to 1.5 percent, one of the industry's two key revenue streams.
The other cash driver is a 20 percent commission on investment returns, known as carried interest, although there is typically a three to five year lag between fundraising and those returns filtering through to the bottom line.
Analysts see some private equity firms approaching so-called harvesting phases from funds raised between 2012 and 2014.
Blackstone shares were up 1.5 percent in morning trading, while Apollo shares were up 3 percent. (Reporting by Joshua Franklin in New York; Editing by Meredith Mazzilli)