The growth in various types of industries together with population growth has resulted in enormous increase in production of various types of waste plastic materials worldwide. This creates a problem on the disposal mechanism. To deal with the problem, study on use of plastic waste as partial replacement to bitumen in flexible pavement is considered in the present work. This study examines the effect of blending waste thermoplastic polymers, namely Low density polyethylene (LDPE) and Polyethylene terephthalate (PET) in conventional80100 ⁄ graded bitumen, at different plastic compositions. Theplastics were chopped in size of 1 -5mm and blended with the bitumen, with a shear mixing of 10003000rev/min and temperature range of 150-180 ºC. Optimizations of mixing parameters were done using central composite design coupled with surface response methodology in design expert version 7.0.0 software. Optimal results of PET mixed modified bitumen size 3mm, composition 8%, temperature 160oc and mixing rpm of 1099.6rev/min, whereas for LDPE mixed modified bitumen size of 2.8mm, composition of 7.39%, temperature of 160oC and mixing rpm 1062.02rev/min are achieved. From the experimental result it was observed that Marshall Stability and softening value of both PET and LDPE modified bitumen increased as compare with conventional bitumen whereas, from penetration and ductility values measurements for both PET and LDPE mixed modified Bitumen decreases as compare to conventional bitumen this indicating that, it increased strength of modified bitumen. From this study it concluded that, by adding waste LDPE and PET plastics into bitumen it significantly improved the quality of modified bitumen. Using PET and LDPE modified bituminous mix also contribute to the recirculation of plastic wastes to another valuable product, as well as to protect the environment.