Election of Sarbananda Sonowal is a formality as he was declared the party's chief ministerial candidate before the polls and led it to a stupendous maiden victory in the north eastern state.
A meeting of the party's Parliamentary Board, which was attended by Prime Minister Narendra Modi and BJP chief Amit Shah, among others, appointed the two senior leaders as observers.
The legislature party meeting is likely to take place next week and BJP sources said the oath-taking ceremony will be a grand affair with all top leaders, including Modi and its chief ministers, likely to attend.
The leadership of Modi and Shah also came in for praise, party leaders said.
BJP has on its own won 60 of the 126 seats in the Assam assembly, while its two allies emerged victorious in 26 seats.