Joan B. Kroc, widow of the McDonald's fast-food magnate, announced yesterday that she has given the Democrats $1 million, the biggest single contribution in the party's history.

Kroc, a registered independent and an advocate of nuclear disarmament, said the contribution was motivated by concern about the direction the nation has taken during the Reagan administration.

"In the name of 'national security,' we have accelerated the buildup of weapons. We have diverted our bountiful resources from improving our lives and our environment to an unwarranted and excessive increase of our military weapons," she said.

Kroc said she has also given the maximum $1,000 to each of the Democratic presidential contenders who has a campaign committee, and she expects to line up behind one of them before the party's nominating convention next summer.

Democratic National Committee spokesman Terry Michael said the contribution came about when DNC Chairman Paul G. Kirk Jr. telephoned Kroc, who lives in La Jolla, Calif., to seek her support.

"I said to Paul, 'Well, of course, I'd like to help' . . . I said put me down for $1 million," Kroc said in an interview.

Since there is a $25,000 election law limit on the amount an individual can give toward federal campaigns, the money will go to state elections, the DNC said.

"This is the largest contribution in the 139-year history of the

Democratic National Committee," committee spokesman Michael

said.

Kroc inherited the McDonald's fortune from her husband, Ray Kroc, founder of the fast-food empire, who died in 1984. He was a major contributor to Republicans, notably Richard M. Nixon.