Protem Speaker Kameng Dolo administered the oath to 58 members in the 60-member house.
Chief Minister Nabam Tuki, who was sworn-in on May 18, was the first to take the oath followed by other senior ministers and MLAs.
Congress MLA from Chowkham constituency CT Mein could not take the oath or affirmation on health grounds.
Election of the new assembly speaker and deputy speaker would be held on Tuesday.
Tuki expanded his ministry on Sunday night by inducting nine Cabinet ministers who were administered oath and secrecy of office by Governor Lt Gen (Retd) Nirbhay Sharma.
The Congress had won 42 seats, including 11 uncontested and one posthumously in the April 9 assembly elections, while opposition BJP secured 11 seats, Peoples Party of Arunachal five and Independents two seats.
Kanubari constituency Congress candidate Newlai Tinghatra died in a private hospital at Gurgaon on May 7.


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