In this research work, reactive mixing of PS and high density polyethylene (HDPE) was carried out in HAAKE Rheocord mixer with an organic peroxide initiator, strength enhancement of the blends can be achieved. A two step mixing method was developed. Effects of the factors of the second mixing on the impact strength of the blends, differences between the two step mixing method and the one step mixing method in rheological behavior, thermal properties of the blend were studied.