Scrap tires and plastics are among the main solid wastes generated in large amount throughout the world. Polyethylene Terephthalate (PET) is the most consumed and wasted plastic. In this study the effects of PET particles size and content on some engineering properties of asphalt concrete containing rubber modified asphalt has been investigated. Different concentrations of waste PET including, 0, 2, 4, 6, 8 and 10% (by the weight of binder) in two different ranges of size were to asphalt concrete and the Marshall stability, Marshall quotient, indirect tensile strength (ITS) and uniaxial dynamic creep properties were evaluated. Results show that the Marshall stability and Marshall quotient increase with increasing PET content. The indirect tensile strength (ITS) test results show that the highest ITS is obtained by adding 2% of PET into the mixtures and beyond that the ITS decreases with increasing PET content. Dynamic creep test results revealed that the resistance against permanent deformation decreases with increasing PET content. However, the mixture containing fine graded PET particles has more resistance against permanent deformation than the mixture containing coarse graded PET particles. Comparing the results on the rubber modified mixtures in this research with those on conventional mixtures accomplished previously reveals that the trend is different.