The executive board of the National Treasury Employees Union has directed the union's 70,000 members to boycott this year's Combined Federal Campaign because the National Right to Work Legal Defense and Education Foundation is participating in the giant fund drive.

NTEU, with 7,000 members here, becomes the second federal union to urge its members not to sign CFC payroll deduction cards this year.

Last month the National Association of Letter Carriers convention voted to urge its 250,000 members to boycott the CFC for the same reason.

In 1981 the CFC, which takes voluntary donations from federal and military personnel, raised $87 million for 65 participating charities and organizations. Federal and military people in the Washington area pledged $13.8 million to the CFC.

This year, under broader guidelines, the CFC drive covers about 120 groups. New members include organizations such as the Sierra Club, Children's Defense Fund, Center for Auto Safety and NRW legal fund.

Unions are furious over the inclusion of the NRW group--an affiliate of the National Right to Work Committee--which they consider to be violently anti-union. (The National Right to Work Committee says it is not anti-union, but simply opposes compulsory unionism.)

The CFC also includes "charities" at opposite ends of the spectrum, ranging from National Right to Life to Planned Parenthood.

The AFL-CIO has taken the position that members should participate in the fund drive, but should carefully designate contributions so they go to groups that are not offensive.

A spokesman for the Office of Personnel Management said only .3 percent of last year's CFC contributions were undesignated. He said the emphasis this year is to get federal and military personnel to designate which group is to get their contributions.

NTEU President Vincent L. Connery said his union is urging members to give "independently to the charity of their choice" but not to participate in the payroll deduction plan of the CFC. Connery said he was "appalled that this antilabor group . . . would be allowed to receive charitable contributions" which are tax deductible.

An OPM spokesman said his agency had to allow more new groups into the CFC because of a court order last year which permitted the National Organization for Women Legal Defense Fund to participate in this year's CFC.

Letter Carriers President Vincent Sombrotto says he tried, unsuccessfully, to get his convention to adopt the AFL-CIO position urging members to participate in the CFC but to designate donations. "I have no beef with the CFC," Sombrotto said. "Our relations with them have always been good and our people have a solid record of participating."

Sombrotto said, however, that he objects to the politicization of the CFC by including "groups that are political, and in no way could be called charities."

Sombrotto said he hopes the boycott actions this year will force the government to get the CFC "back in the business of representing bona fide charities" and not serving as a collection agent for "groups that are clearly political, whether you favor their goals or not."