The board of trustees of the New National Theatre Corp. (NNTC) voted yesterday to accept a "$1 million plus" interest-free loan from the New York-based Shubert Organization. In return, NNTC has revised its existing agreement with the Shuberts, giving them a 20-year contract to book the historic theater.

The loan will be used to redecorate the National, which is closed for renovation until the fall of 1983.

The Shuberts were originally under contract in 1980 to book the National for five years. The new contract, which takes effect immediately, can be terminated only if they book consistently inferior shows or fail to provide any at all.

"Obviously, 20 years is a long time, and quality assurance is one of our main concerns for the National," Harry Teter Jr., president of the NNTC, said yesterday. "This is one of the most far-reaching decisions we've had to make concerning the theater. But we believe the Shuberts are the foremost theatrical organization in New York today and there's no reason to think they won't continue to be in the future."

Teter called the new arrangement "the finest deal that could be gotten for the National Theatre."

Under the complex agreements governing the National Theatre, Quadrangle-Marriott, a local development corporation, is responsible for the structural improvements currently under way. Estimated at $4 million, they include expanded lobbies and restrooms, new dressing rooms and a new heating and airconditioning system.

Improvements in interior decoration, however, are the responsibility of NNTC, the nonprofit organization that has run the National since 1976 and that has had severe financial problems in the past. "There was no way of financing a $1 million loan on our own. Our annual payments would have been in excess of $200,000 a year," Teter said. The board also decided that launching a massive fund-raising campaign to cover the decorating costs would not be "a viable option."

Under the new contract, the Shuberts will continue to pay NNTC $100,000 a year for the next 20 years in exchange for the right to book the theater. Teter explained that the $1 million interest-free loan will be repaid out of profits from shows playing the National over the next 10 years. Since NNTC and the Shuberts split those profits on a 50-50 basis, the Shuberts will actually be paying for half the cost of decoration, he said.

In addition, the Shuberts have agreed to give NNTC the first $76,000 of annual concession profits, plus 50 percent of all profits over that amount.

The new contract mends a dispute that, as recently as spring of this year, had the NNTC and the Shuberts treating one another as virtual enemies. In June, the NNTC board elected Teter to replace Maurice B. Tobin as its president, and relations have improved steadily since then. Both the Shuberts and NNTC now say they are working hand in hand to turn the National into a theater that will rival the Kennedy Center in elegance.

The redecoration of the facility will be undertaken by Pat Rosen, assistant to the director of the Shubert Organization, who is in charge of the design and decoration of the 21 1/2 theaters in the Shuberts' nation-wide empire. (The organization owns half interest in the Music Box in New York.)

"We won't be changing the character of the space at all," Rosen said recently. "We're just enhancing it, enriching it with color and fabric. The entire theater will be wrapped in deep blue and gold." Plans call for new seats, carpeting, curtains, lighting fixtures, wall coverings and two chandeliers in the auditorium. The gold-leaf eagle, which adorns the proscenium, will become a motif throughout the theater, beginning with the carpet in the entrance lobby.

"The redecorated National will have a very regal look," Rosen said. "It will be nothing like any theater we have in New York."

NNTC's board first considered the Shubert offer at a meeting two weeks ago, but withheld its final decision until yesterday, when copies of the contract were made available to members.