For the first time in six years, the residential mortgage loans owned by the nation's savings and loan associations are worth more than what the associations invested in the loans, the Federal Home Loan Bank Board reported yesterday.

Bank board officials and thrift industry economists said the main reason for the dramatic rise in the market value of the loans is the recent decline in interest rates. As a result, the gap has narrowed between the rates thrifts must pay to borrow money and the yields they receive on mortgage loans. The narrowing of these "spreads" makes the mortgage loans more valuable.

Edwin J. Gray, chairman of the bank board, which regulates federally insured thrifts, said the figures show "clearly that the prospects for very substantially improved prosperity in the savings-institutions industry are dramatically better today than when the decade began."

He cautioned, however, that despite the gains, the thrift industry "is not moving as quickly as it should in reducing its exposure to interest-rate surges in the future."

The figures released yesterday represent the culmination of many months of improvement in the value of the mortgage portfolios of savings associations as a result of the decline in interest rates. Thrifts had chafed under the high interest rates of the early 1980s, which had dampened profits and precipitated financial difficulties at many savings institutions. At the worst times during this period, the market value of residential mortgages was less than 75 percent of their book value, said Eric I. Hemel, director of the bank board's Office of Policy and Economic Research.

But since mid-1984, interest rates have plummeted more than 4 percentage points -- 1 1/2 percent in the last three months alone -- adding tens of billions of dollars to the market value of the industry's mortgage portfolio, the bank board reported. Hemel said bank board figures, based on reports from each federally insured savings and loan, show that at some point in the past two months, the market value of the mortgages exceeded their current book value of $675 billion.

"We can now say definitively that the thrift industry has passed the threshhold where its mortgage portfolio, in the aggregate, could be sold today at a gain," Hemel said.

Thrift industry economists hailed the bank board's news as a further sign of the industry's general turnaround, although they said certain individual institutions continue to face difficulties because of bad real estate and construction loans.

"If the study is correct in its conclusions, it has a very great deal of significance," said Jonathan E. Gray, who follows the industry for Sanford C. Bernstein & Co. in New York. "It would suggest that the net worth of the industry is understated."

The value of the net worth of the nation's thrifts, that is, assets minus liabilities, has been a deep source of concern to the regulators, who must bail out institutions whose net worth dips too low. In particular, critics have questioned the adequacy of the insurance fund controlled by the bank board to help the troubled thrifts. Gray suggested, however, that if the industry's net worth is in fact higher than anticipated, the level of funds the industry must contribute to the fund could be reduced.

Gray said the decline in interest rates is likely to contribute to a year of record earnings for the industry. He projected that thrifts are likely to show a profit of at least $6 billion this year, compared with last year's break-even results.

Thomas D. Klingenstein, an analyst with Wertheim & Co., agreed that the board's report was good news, but said it came as no surprise given the trends in the industry. He speculated that the primary reason for its release was to counteract recent bad publicity. "Most of the things you read about the thrift industry are negative," he said. The report's release "is primarily a public relations effort."

Nonetheless, Klingenstein and other analysts said the current economic environment presents the industry with an opportunity to restructure mortgage loan portfolios and to reduce exposure to big losses if the economy declined.

They said that savings and loans are increasingly trying to sell fixed-rate mortgages in favor of holding other types of financial instruments less prone to interest-rate movement. For instance, of the $675 billion total book value of the mortgage portfolios of U.S. thrifts, about $225 billion is now in the form of adjustable-rate mortgages, which are tied to various fluctuating economic indices, Hemel said.

Gray expressed concern that many institutions have not done as much as possibile to reduce their vulnerability. He warned that these institutions "will, as history demonstrated in 1981 and 1982, likely not survive when such a scenario reoccurs."