Former communist rebels joined the interim government as part of the landmark peace deal that ended a decade-long insurgency, pledging to ensure development in the Himalayan nation and to hold credible elections.
Five former Maoists were appointed ministers, while a sixth was given a junior minister portfolio in the new government to be headed by Girija Prasad Koirala.
The swearing-in ceremony was broadcast live on national television.
The former insurgents were sworn in along with 20 other ministers and junior ministers by a visibly pleased Koirala, 84, whom parliament re-elected as prime minister earlier Sunday for a sixth time.
Maoist leader Prachanda said their main focus would be to conduct elections for a special assembly that will rewrite the constitution and decide whether Nepal will continue as a constitutional monarchy state.
"This day marks the beginning of a new Nepal. Our priority for now is to hold the elections, which will be free and fair," Prachanda said, whose real name is Pushpa Kamal Dahal.
"Our next goal is to provide some immediate relief to the people and then turn toward long-term development of the country."
The inclusion of the former rebels is part of a peace process that began last year when the Maoists agreed to give up their armed campaign to rid Nepal of its constitutional monarchy and join an interim government.
The landmark agreement on the formation of a joint government was finalised late Friday in a meeting involving Koirala, leaders of the seven ruling coalition parties, and Prachanda, the leader of the former rebels who goes by a single name.