A 29-year-old Alexandria unwed father yesterday won custody of his 2 1/2-year-old daughter after he told a Virginia judge that the child's mother repeatedly frustrated his efforts to visit the child.

"I'm beaming all over," said the father, John Younger, a Northern Virginian tow truck operator, after the court decision.

The ruling by Alexandria Domestic Relations Court Judge Joseph L. Peters Jr., was the second in a week in Northern Virgina that took a hard line against parents who try to thwart judicial orders granting child visitation rights.

Last week a Fairfax County jury awarded $25,000 in damages to a father claimed his divorced wife had prevented him from visiting their three daughters.

In yesterday's case, the Alexandria judge sided with Younger's arguments that he should be granded custody of his daughter who had been living in Southside Virginia with her mother.

"I'm not trying to keep her from her mother," Younger said, "I only wanted to be able to see her but her mother wouldn't let me."

Until recently, courts have been reluctant to enforce visitation agreements when one parent refuses to co-operate, according to various groups representing divorced fathers.

Judge Peter's decision yesterday was described s "the logical extension of the rights" of fathers, said Dorris Jonas Freed, the American Bar Association's top expert on custody issues. "They (the unwed fathers) now stand substantially in the shoes of wedded fathers."

Virginia State Sen. Adelard L. Brault, whose Fairfax City law firm handles many custody cases, said yesterday he couldn't recall another case in the area in which an unwed father had sought, much less won, custody of his child. "There's no question, this is an unusual case," he said.

Judge Peter's decision cannot be used as a precedent in Virginia or elsewhere because his court is not a court of record. The losing mother, Betty Jean Cheek, 23, has appealed the decision to Alexandria Circuit Court, which could set a precedent.

Younger's victory came after two years of unsuccessful efforts to see his daughter -- a struggle that led him last June 16, one day after Father's Day, to journey to Haifax, Va., and seize the child as she walked to the mailbox with her mother.

"It was the only thing I had left," Younger said. "I kept doing what the court asked. But the court couldn't do anything for me. Finally I did something wrong to do something right."

Younger was charged with both kidnapping and assault and battery, but was acquitted when a judge ruled he did what he did in an effort to see his daughter, Tonya, under a court order.

After seizing Tonya in Halifax, Younger brought her to Alexandria and enrolled her in Green Apple Day Nursery.

Younger said he and Cheek lived together in Halifax and Alexandria but split up after pitched arguments. In September 1978, Cheek won custody of Tonya, and Younger received the right to visit his daughter for seven hours on the second and fourth Sunday of each month in Halifax, Cheek's hometown.

Under the order, Younger said, he was to pick up his daughter at the Five Forks Baptist Church at 10 o'clock Sunday morning. "But she (Cheek) never showed up," he said.

Younger said he journeyed to Haifax in Southside, just north of South Boston, "10 or 15 times" but was never able to see Tonya -- until he seized her from her mother.