Soviet nuclear scientist and human rights champion Andrei Sakharov, making his first visit outside his homeland in 30 years, arrived in the United States urging both countries to "make the world better without war."

Sakharov, who plans to visit relatives, seek medical treatment and meet with President Reagan on his first trip to the United States, was to meet first Monday with members of the American Academy of Arts and Sciences in suburban Somerville, Mass.The frail, 67-year-old father of the Soviet hydrogen bomb, arriving in Boston on Sunday night after a 91/2-hour journey from Moscow that included a brief stopover in New York, was greeted by his stepdaughter, Tatiana Yankele-vich, her husband, Efrem, and their two children.

While in New York awaiting his flight to Boston, he said through an interpreter, "I'm delighted to be here in the U.S. This is a great country with a great future. Together with our country, we must make the world better without war."

And at a brief news conference at Boston's Logan International Airport, the 1975 Nobel Peace laureate called the United States "a very great country."

"I also think my country is a great country, and together we can do a great deal to preserve peace on the Earth and put an end to hunger and violence," a visibly fatigued Sakharov said, his stepdaughter translating.

Yankelevich, daughter of Sak-harov's wife, Yelena Bonner, from a previous marriage, lives in the Boston suburb of Newton, where Sakharov will stay during the Boston leg of his trip.

Bonner did not accompany Sakharov because of illness and told UPI in Moscow that she could not discuss "his program in the United States."

Sakharov spent 7 1/2 years in internal exile in the Soviet city of Gorky for criticizing the Soviet invasion of Afghanistan. He was freed in 1986. "Two years ago, I became a free man when I returned to Moscow," Sakharov said. "Today I became even freer because I was granted the right to travel around the world."

Despite many invitations to travel abroad since his release, he had been refused permission to leave the Soviet Union on the grounds he knew state secrets.

Sakharov credited imprisoned fellow dissident Vazif Meilanov with helping him gain permission to make his U.S. visit. "I am here because of this man's demonstration in my defense, and he is not free," Sakharov said.

Meilanov is being held in a rural Soviet prison on charges of criticizing Soviet human rights violations, according to a representative of Amnesty International, who attended the news conference.

Dr. Jerome Wiessner, president emeritus of the Massachusetts Institute of Technology, who was among those greeting Sakharov, said the Soviet scientist would be seeking unspecified medical treatment in the United States.

Sakharov is expected to seek medical care at Massachusetts General Hospital in Boston, where Bonner had cardiac bypass surgery in January 1986, The Boston Globe reported last week, citing anonymous sources.

Sakharov likely will be examined for possible implantation of a cardiac pacemaker, a small device inserted under the skin of the chest that regulates heart rhythm. The implantation is performed under local anesthesia and usually requires about a two-day hospital stay, the newspaper said.