Besides, the recent floods have affected over 350,000 people, officials said on Tuesday.
Over the past two weeks, more than 150 people were killed in similar strikes in the state, with 350,387 people having been affected by the floods.
Disaster management officials said the victims were either working on their fields or had taken shelter under trees at the time of lightning.
The floods have inundated Sitamarhi, Sheohar, Supaul, Kishanganj, Darbhanga, Muzaffarpur, Gopalganj and East Champaran.
Authorities have set up relief camps in Supaul and Gopalganj for the people, and reports said people in other affected districts have taken shelter with their friends and relatives.
Chief Minister Nitish Kumar on Monday directed disaster management department to remain fully prepared in view of heavy rains in the catchment areas of neighbouring Nepal and Gandak river.
Officials said water in Ganga, Baghmati, Kamlabalan and Mahananda rivers was flowing above the danger level at several places. Floods have also ravaged northeastern state of Assam where 24 districts are inundated.