In order to adapt LaSota strain of Newcastle disease virus (NDV) on chick embryo fibroblast (CEF) cells, 0.1 ml from LaSota vaccine produced in Razi vaccine and serum research institute inoculated to the CEF cells and was passaged five times in the CEF grown in Eagle’s minimum essential medium (MEM). First to third passages were blind but in fourth passage cytopathic effect of virus was observed. Fifth passage viruses were propagated in SPF embryoneted eggs and then harvested allantoic fluids showed presence of NDV with the titer of 108 EID50/ml. Plaque assays performed on this harvest using two agar overlays one supplemented with trypsin, DEAE dextran and magnesium sulfate, the second with neutral red along with the mentioned supplements, overlaid 72 hours after first one. Eight hours later, plaques of adapted virus appeared with various sizes ranging from 1 mm to 3 mm in the diameter. Discrete plaques were observed in the 10-6 dilution and the calculated titer of virus was 3×107 pfu/ml. Agar medium overlays without above mentioned supplements obtained no plaque at any dilutions.