Former Rwandan army chief Augustin Bizimungu has been sentenced to 30 years in prison for his role in the 1994 genocide.
The UN war crimes tribunal for Rwanda also convicted ex-paramilitary police chief Augustin Ndindiliyimana but released him for time already served.
Two other senior generals were each sentenced to 20 years in prison.
Some 800,000 ethnic Tutsis and moderate Hutus were killed in the 100-day genocide.
Bizimungu and Ndindiliyimana are two of the most senior figures to be sentenced by the International Criminal Tribunal for Rwanda (ICTR), established in Arusha in neighbouring Tanzania to try the ringleaders behind the killings.
Belgian peacekeepers killed
"It is a welcome decision by the ICTR. In its own circumstances, that is a big sentence, even if many people would think he [Bizimungu] deserved the highest," Martin Ngoga, Rwanda's chief prosecutor, told Reuters news agency.
The court ruled that Bizimungu, who was arrested in Angola in 2002, had complete control over the men he commanded in 1994, AFP news agency said.
Ndindiliyimana, however, was said to have only had "limited control" over his forces and was described as being opposed to the killing.
Having already spent 11 years in jail following his arrest in Belgium in 2000, Ndindiliyimana was released.
Both men were found guilty of genocide and crimes against humanity.
The BBC's Will Ross in Nairobi, Kenya, says Bizimungu appeared unmoved when he was handed his sentence.
The 59-year-old was accused of going to the homes of militants and ordering them to kill all those from the Tutsi ethnic group - people he referred to as cockroaches.
He was said to have promised weapons, as well as fuel to burn houses, our correspondent says.
Major Francois-Xavier Nzuwonemeye, the former commander of a reconnaissance battalion, and his second-in-command, Capt Innocent Sagahutu, were meanwhile each given 20-year sentences for crimes against humanity.
They were accused of ordering the murder of Prime Minister Agathe Uwilingiyimana.
Eight Belgian peacekeepers who were protecting the prime minister were also killed, triggering the withdrawal of the UN force from Rwanda.
Rwanda's genocide was sparked by the death of former President Juvenal Habyarimana who was killed when his plane was shot down close to the capital, Kigali, on 6 April 1994.
Within hours of the attack, certain members of the government organised Hutu militias across the country to systematically kill Tutsis, resulting in more than three months of violence.
The Hutu government blamed Tutsi RPF rebels for killing Mr Habyarimana but RPF leader Paul Kagame, now Rwanda's president, says the plane was shot down to provide a pretext for the premeditated slaughter.
Many thousands of lower-ranking people accused of involvement in the genocide have been put on trial in Rwanda, either in formal courts or in a traditional system known as "gacaca".