Beginning tomorrow morning, the Corcoran Gallery of Art will stop charging for admission.

Dr. Armand Hammer, the billionaire art patron who yesterday pledged to give the private gallery more than $1 million, said most of the money-$900,000-will be used to offset entry fees.

The Corcoran instituted a pay policy with its establishment in 1869. In recent years, the museum has charged most of its visitors $1.50 each. But under its new policy the public will be admitted free of charge throughout the week.

The remainder of the Hammer gift-$250,000-will be used to air-condition, redecorate and reequip the Corcoran's auditorium.

In addition, Hammer's monies may be matched, in part, by the National Endowment for the Arts.

Encouraged by his promise, the newly confident museum, which dared not do so in the past, has applied to the Endowment for a three-year $750,000 "challenge grant." Should the cash come through, the Corcoran, to qualify, would have to raise $2.4 million in matching funds feom entirely "new sources." Thanks to Hammer's pledge, it already has nearly half that sum in hand.

Hammer said yesterday his foundation will deliver the $1,150,000 to the Corcoran in 10 equal annual installments. He also hinted he will give more to the museum in the years to come.

Hammer, the board chairman and chief executive officer of the Accidental Petroleum Corp., has been comparably generous to two other art museums, the Los angeles County Museum and the National Gallery of Art. He has willed his paintings to Los Angeles; his Old Master drawings will be left to the National Gallery. A longtime confidant of the leaders of the Soviet Union, Hammer has arranged for the United States, numerous loans of art from Russian state museums. One such loan show, Old Masters from the Hermitage-one a Leonardo, will open here May 13.

The Corcoran donation was announced at a ceremonial unveiling held between the drowsing bronze lions on the gallery's front steps.Effi Barry, the mayor's wife, spoke briefly to the crowd. "Economics should not be a deterrent to viewing works of art," she said. A Marine Corps color guard presented flags, a boy's choir from Houston sang, and Livingston Biddle, the Endowment's chairman, David Lloyd Kreeger, the Corcoran's board president (at whose invitation Hammer six months ago joined the board) and Corcoran director Peter Marzio said thank-you to the doctor, who is non-practicing physician. When their speeches ended, a black veil was removed from the new marble tablet set beside the door. In gilded letters large enough to be read across the street it spells out the word "FREE."