In this paper, the optimum composition of fly ash and plastic waste in a composite has been established based on various performance tests. The optimum dose of plastic waste in composite is 8% weight of fly ash. Laboratory tests indicated increased resistance to moisture damage in stone Matrix Asphalt (SMA) mixture containing composite as filler. The values of resilient modulus of the SMA mixture containing composite are fairly high compared to SMA mixture containing lime as filler. The rutting in SMA mixture containing composite is reduced to one third as compared to conventional SMA mixture without composite. The results of mechanistic analysis indicate that there is an increase in allowable number of traffic on modification of SMA with fly ash – plastic waste composite as filler in place of lime.