In this study, the effects of fly ash in composites fabricated by injection molding are examined. Taguchi design of experiment was first utilized to estimate the effects different injection molding conditions and content ratios of fly ash have on a linear low-density polyethylene (LLDPE)-fly ash composite. The results reveal that the content of fly ash is highly significant and contributive to the shrinkage ratio and bending strength. For these reasons, LLDPE and polypropylene (PP) composites with different size particles of fly ash were fabricated and the mechanical properties were investigated. The particle size was changed by grinding fly ash with a planetarium ball mill. The shrinkage ratio, bending strength and flexural modulus of LLDPE composites containing raw fly ash were found to improve. The shrinkage ratio and flexural modulus of PP composites containing ground fly ash were also found to improve. Homogenization analysis using the finite element method was then used to calculate the Von Mises stress distributions and homogenized elastic matrix of PP composites containing ground fly ash. The homogenized elastic matrix was used to validate the experimental flexural modulus. The results show that the homogenized elastic matrix is in good agreement with the experimental flexural modulus. POLYM. COMPOS., 2012. (c) 2012 Society of Plastics Engineers