Fly ash (FA)-general purpose unsaturated polyester resin (GPR) particulate composites have been made. The effect of surface treatment of FA with two different silane coupling agents (CAs) on the mechanical properties like tensile, flexural, impact strength and hardness, thermal properties like thermal stability and morphological properties (SEM) of FA-GPR composites are studied. The properties of FA-CA-GPR are also compared with that of GPR and CaCO3-GPR. An enhancement in the tensile, flexural, and impact strength and moduli are observed when FA is surface treated with CA. Hardness is also found to increase with CA-treated FA-filled GPR. A suitable mechanism for the chemical reaction taking place at the interface in the presence of CAs is proposed.