The present invention relates to nano sized sulfide compounds of cerium and a process for the preparation thereof. More particularly, the present invention provides novel nano sized particles of cerium sulfide as well as a novel process for the preparation of nano sized sulfide compounds of cerium by the bioreduction of cerium sulfate or cerium acetate, without resorting to chemical methods. A bioprocess is disclosed to treat trivalent Cerium salt with sulfate-reducing bacteria (SRB) under controlled conditions to obtain a biomass, which is subjected to staggered heating upto a temperature of 600-1500.degree. C. The sulfide of Cerium is finally separated for application in pigment industry.