Flyash, a waste product of thermal power stations which is generated in huge quantities, poses problems in its disposal. It contains a variety of inorganic oxides and is available in finely powdered form. Attempts have been made for its utilisation as a filler in elastomers and plastics. It is important to note that flyash in untreated form does not contribute in enhancing mechanical properties of composites significantly. In this paper, flyash treated with silane coupling agent (Si-69@) was studied as a filler in polybutadiene rubber. A comparison of properties of composites filled with treated and untreated flyash revealed that the composites with treated flyash exhibited better properties. Flyash treated with silane coupling agent gave greater reinforcement than untreated flyash. The treatment resulted in the enhancement of mechanical properties of composites of the said rubber. In this study, properties of composites filled with treated and untreated flyash were compared. The properties studied were tensile strength, modulus at 100;and 400;, Youngs modulus and hardness. Tensile strength was improved by 193;, modulus at 400;improved by 700;and Youngs modulus improved by 170;.