(Adds Goldman letter on fund's performance)

By Jonathan Stempel

NEW YORK, April 13 (Reuters) - Goldman Sachs Group Inc's (GS.N: Quote, Profile, Research) asset management unit on Monday said it has raised $5.5 billion from investors to start a fifth fund dedicated to buying private equity investments on the secondary market.

The new fund, GS Vintage Fund V, follows earlier funds in which Goldman raised close to $6 billion in total. The most recent was in March 2007, when Goldman said it raised $3 billion.

But in a letter to investors obtained by Thomson Reuters unit peHUB, Goldman said it may have to write down some of the fund's early investments, reflecting an accounting change, currency fluctuations and a "severe decline" in the value of comparable investments amid a "difficult" financial and economic environment. A writedown may total 5 percent to 9 percent of the portfolio, peHUB said.

A Goldman spokeswoman had no immediate comment.

Investors are forming funds to buy secondary interests as financing remains scarce, with the global credit crisis approaching its second anniversary. Goldman's new secondary fund is the largest of its type, the Financial Times said.

"Since late 2008, there has been a lot of interest" in secondary market transactions, said Colin Blaydon, a professor at Dartmouth College's Tuck School of Business and director of its Center for Private Equity and Entrepreneurship. "As long as the economy and the capital markets struggle, there will be a fertile market for secondaries."

Goldman expects the new fund to buy portfolios of private equity assets, including limited partnership interests in private equity funds, with transactions ranging from $1 million to more than $1 billion.