The only Vietnamese national assemblyman to serve under both the American-backed government and victorous Communist regime testified yesterday that 700 persons have been executed and more than 6,000 imprisoned in his home province lone since the fall of Saigon.

The former parliamentarian, Nguyen Cong Hoan, told the House International Relations Subcommittee on International Organizations that he obtained that information from trends and relatives and from sources available to him as a government official.

Hoan fled the country late in March in a fishing boat and took refuge in Japan. He was permitted to enter the United States last week end, after the House subcommittee requested his testimony, and hopes to remain here as a refugee.

Hoan, speaking through an interpreter, said there are seven camps for political prisoners in his home province of Phu Yen, which is about half-way up the Vietnamese coast between Saigon and Danang. He said that some claim ot be "re-education camps" but actually are prisons for those who served the old regime or are otherwise deemed unreliable.

One of the prisons, he said, houses about 1,000 persons who fled Vietnam in the U.S. - sponsored evacuation on April 1, 1975, but who were repatriated from Guam late that year at their own request.

About 500 persons were executed in the immediate aftermath of the Communist victory. Hoan testified, and about 200 have been killed since. After the hearing, he told reporters that a principal source of his information was the former Communist Party military chief in the province, who has since been deposed.

An anti-government deputy who was close to the Buddhist political movement during the regime of President Nguyen Van Thieu, Hoan was elected to the unified Vietnamese assembly after the takeover and served for about a year. He testified that he was selected to run for the unified assembly by a local Communist committee, although he said he was not a party member.

In fleeing Vietnam he left behind his mother, wife and four childran. He said he decided to leave the country, despite hardship to himself and his family, in order to bring out the story of what is happening.

At several points during his questioning, Hoan disavowed prepared remarks in English that were read for him to the committee. One statement, he said had been based on press reports in a Vietnamese language newspaper in the United States and not on his own knowledge.

Hoan said in his prepared statement that he heard "news" about Americans missing in action while he was in Hanoi to attend the legislature. In answers to questions, howere, he said he heard "some rumors" on the subject, but declined to divulge any data in public session.

After two years under the Communist regime, most Vietnamese realize they have much less individual freedom than before. Hoan maintained. In his province, he said. "every family" has some relatives in prison camps.