Angry over not getting a buffalo in dowry, Upendra Mahto poured kerosene on his wife Rina Devi (25) and 14-month-old son at their house in Nashrathpur village, police said, adding the mother-son duo were charred to death.
The bodies have been sent for post-mortem, they said.
While Mahto has been arrested, three other accused persons named in the FIR by the woman's kin were absconding.
Raids were being carried out to nab those accused who are at large, they said.