A U.S. judge ruled here yesterday that the immigration and Naturalization Service has no right to try to deport the foreign-born spouse of an American citizen merely because the couple has encourtered marital difficulty and informally separated.

U.S. District Court Judge Harold H. Greene blocked the immigration agency from trying to deport Ah Kow Chan, a Malaysian national who is married to a Montgomery County Woman, Laurel Allen Chan.

The Chans met while attending school at Wesleyan College in Athens, Tenn., and got married in January 1975 after a lenghy courtship. Six months later they separated and then the INS denied Ah Kow Chan's bid to declare him an "immediate relative" of an American. Such a designation would have entitled him to become an American citizen.

The immigration agency's Board of Immigration Appeals ruled last year that the Chans' marriage was "nonviable," even though the couple has never instituted legal separation or divorce proceedings and still hold out the hope of a reconciliation.

Greene said that "no rule or regulation issued by the attorney general or the Immigration and Naturalization Service requires the existence of a 'viable' marriage" as a precondition to the grant of immediate relative status.

"The Immigration and Naturalization Service clearly could not, consistenly with due process of law, be regarded as vested with both the authority to establish the vague and elusive concept of marriage viability and the enormous power to regulate and enforce that concept in actual practice," Green said.

"INS has no expertise in the field of predicting the stability and growth potential of marriages-if indeed anyone does-and it surely has no business operating in that field," the judge ruled.

"I can appreciate the situation where you don't want Americans selling marriages" to gain U.S. citizenship for the foreign-born spouse, said William J. Chen Jr., the Chans' lawyer. "But there was absolutely no challenge" to the legitimacy of the Chans' marriage.

Chen said Laurel Chan 27, is working as a bookstore official in the Washington area, while her husband, 31, runs a fast-food restaurant in Athens, Tenn. Both, he said, are working toward graduate degrees.