Ballerina Cynthia Gregory announced yesterday that she is leaving American Ballet Theater, primarily because the company has no male partner she considers suitable to her stature and artistic needs.

It is the second time in two years she has dropped out. In December of 1975, Gregory announced she was "retiring" to get back to her personal life. She went home to California, remarried and learned to drive a car.But a year later she rejoined ABT, making her comeback appearance at the Kennedy Center.

This time, a major factor in her leavetaking is ABT's apparent refusal to import an unknown Romanian dancer she had sought as a partner - a dancer ABT claims neither they nor she has ever seen on stage.

The 31-year old dancer's decision not to sign a contract with ABT for the present season means she will not appear in its four-week season at the Kennedy Center starting Dec. 6.

Gregory told an interviewer in 1975 that the most important things in a ballerina's career were "finding the perfect partner, and inspiring a choreographer to create works on you."

Though Gregory, who is among the tallest of ABT female principals, has appeared alongside such notable dancers as Rudolf Nureyev, Erik Bruhn, Ivan Nagy, Peter Martins, Fernando Bujones and Ted Kivitt, she has never established a durable partnership with a male star of international reputation. Of her other regular ABT partners, only one - Clark Tippet - is still with the company.

Gregory's action was confirmed by her husband, John Hemminger, reached by telephone in the couple's New York apartment last night. Gregory herself was unavailable - "She went out to dinner with a girl friend," Hemminger said, "to escape the phone calls."

Gregory, an extremely strong, clean and flamboyant technician, is widely considered to be one of the finest ballerinas the United States has produced. She enjoyed a great personal triumph in "Swan Lake" in 1967, the year she was named an ABT principal, but her extensive repertoire also includes virtually every major classic role and many contemporary ballets.

In 1974, at the invitation of Alicia Alonso, she became the first American ballerina to dance with Cuba's National Ballet. The following year, Rudolf Nureyev restaged the full-length Petipa classic, "Raymonda," for Gregory, and partnered her in the ABT production.

A short film featuring Gregory and Ivan Nagy, "In a Rehearsal Room," was shown at the 1976 New York Film Festival.

The American Ballet Theater reaction to her departure was to express great disappointment, and leave the door open for an eventual return.