The oath of office was administered by President Ram Baran Yadav and was attended by at least two Communist Party officials despite the party's boycotting of Saturday's parliamentary election won by the new prime minister, Nepalnews.com reported.
The Web site said Nepal skipped the part of the oath that mentions "God" and also did not reference "the people" as did his predecessor Pushpa Kamal Dahal.
Nepal's three-member cabinet was also sworn in, inducting Communist leaders Bidhya Bhandari as defense minister and Surendra Pandey as finance minister.
Nepalnews.com said the Nepali Congress Party said it will name its Cabinet ministers after it reaches an agreement on the number of portfolios to be included. The Communist and Congress parties have reportedly formed separate committee to determine the distribution of portfolios.