Maleated castor oil (MACO) was prepared and was used as biomodifier in unsaturated polyester resin (UPE)/fly ash composites. The prepared MACO was characterized for its molecular weight and viscosity. MACO was blended with UPE in three different ratios like 5, 10 and 15. wt%. Fly ash was incorporated in the blend matrix (10. wt%) and curing was done by free radical polymerization. MACO was polymerized and crosslinked with UPE in situ during the formation of the composites. The cured matrix therefore formed an interpenetrating polymer network and the enhancement in properties was significant. Incorporation of 5. wt% MACO was most effective compared to 10 and 15. wt%, when the impact strength increased by 52% without any loss in modulus. The glass transition temperature also shifted to a higher temperature indicating strong intercomponent bonding in this set of composites. © 2011 Elsevier B.V.