Nitish Kumar had invited Modi to the event.
According to BJP leaders here, two union ministers, M. Venkiah Naidu and Rajiv Pratap Rudy, will represent Modi at the event.
Earlier, Nitish Kumar had invited veteran BJP leader L.K. Advani and BJP Lok Sabha member Shatrughan Sinha to the ceremony.
The event is expected to draw a galaxy of top political leaders including Congress president Sonia Gandhi, Samajwadi Party chief Mulayam Singh Yadav, chief ministers Arvind Kejriwal of Delhi, Mamata Banerjee of West Bengal, Naveen Patnaik of Odisha and Akhilesh Yadav of Uttar Pradesh.
JD-U leaders say the event -- where the star attraction will be RJD chief Lalu Prasad -- will mark a new beginning of opposition unity in the country.
Former prime minister H.D. Deve Gowda is also likely to attend the oath-taking, besides former Jharkhand chief ministers Babulal Marandi and Hemant Soren as well as INLD leader Abhay Chautala.
According to Janata Dal-United leaders, Nitish Kumar will take oath as chief minister along with a 36-member council of ministers.
The Rahstriya Janata Dal (RJD) emerged as the single largest party in the 243-seat house with 80 assembly seats followed by the JD-U (71) and the Congress (27 seats).
There will be 16 ministers from the RJD, 15 from the JD-U and five from the Congress in the new government.