Fighters from the Kamwina Nsapu group attacked a police convoy.
Six policemen who spoke the local Tshiluba were freed, but the rest were killed, Kasai Assembly President Francois Kalamba said.
The unrest in Kasai began last August, when security forces killed the Kamwina Nsapu leader.
Friday's attack targeted a police convoy travelling between Tshikapa and Kananga.
The state Governor Alexis Nkande Myopompa said an investigation had been launched into the killings.