A Fairfax County judge denied yesterday a motion to allow Nancy L. Kantarian, who pleaded guilty 14 months ago to killing her two young daughters, to be released from a private mental hospital.

Kantarian, 32, who has been receiving treatment at the Sheppard and Enoch Pratt Hospital in Towson, Md., has been allowed to leave the facility three or four days a week to work at a women's clothing store in Baltimore and has been out on day and overnight passes to allow for a smooth transition to the community, according to a letter filed in court by her doctor.

After the May 23, 1984, killings of Jamie Lee, 5, and Talia, 6, Nancy Kantarian's psychiatric health was considered so delicate that she was excused from attending her preliminary hearing. Her attorneys said after her sentencing that it could be years before she was well enough to be released.

Commonwealth's Attorney Robert F. Horan asked yesterday where all the doctors were who used words like "chronic," "long-term" and "severe" to describe her condition before her sentencing.

"We oppose release until at such time any of those mental experts who told us how sick she is tell us otherwise," Horan said. None of the psychiatrists who had testified about Kantarian's mental condition were in court yesterday.

Kantarian pleaded guilty Oct. 5, 1984, to two counts of voluntary manslaughter in the deaths of her daughters. Talia was stabbed 32 times and Jamie died of a blow to the head and burns from a fire set by Kantarian at the $400,000 home she shared with her husband and two daughters in Great Falls.

As Kantarian, dressed in a navy blue blouse with a bow at the throat and a gray skirt, watched from the defense table yesterday, attorney Albert J. Ahern Jr. argued that she could derive no further benefit from in-house psychiatric care, although she still needs continual psychiatric counseling. "She has made very, very substantial strides," Ahern said.

Circuit Court Judge Barnard F. Jennings, who sentenced Kantarian in October 1984 to a suspended 20-year sentence and indefinite incarceration in a private mental hospital, denied the motion to delete the part of her sentence requiring in-house hospital treatment.

Since the killings, Kantarian has been under constant psychiatric care and counseling, according to her attorneys. Ahern said the passes Kantarian receives are approved by hospital staff and that she is closely monitored in the hospital, where her stay is paid for by her family.

Horan said of Kantarian's job: "She works in a boutique in Baltimore. Under the rules, she can't be out more than 48 hours . . . . I'm presuming she's following the rules."

Dr. David L. Waltos, in a memorandum filed in support of the motion for his patient's release, said that Kantarian should continue to have individual psychotherapy four times a week.

Kantarian showed no emotion in the courtroom yesterday, but outside in the hall held the hand of her husband, Harry K. Kantarian, a senior partner in the Washington law firm of Housley Goldberg & Kantarian, who kissed his wife. Mr. Kantarian was in Denver on business at the time of his daugters' deaths.

Those who had seen Nancy Kantarian's puffy face during her sentencing last year remarked at the dramatic difference in her appearance yesterday. She comes from a wealthy Rochester, N.Y., family. Her father, John E. Heselden, is deputy chairman of Gannett Co., the Arlington-based media conglomerate that publishes USA Today.