A total of twelve Newcastle disease virus isolates recovered from different outbreaks in Iran were analyzed for in vivo pathogenicity, biological property and amplification of F cleavage site gene. Intracerebral pathogenicity index varied from 1.7 to 1.96 while the intravenous pathogenicity index was more than 2.0. The mean death time value for all the isolates ranged from 39.6 to 68.8 hours. All the isolates except 2, hemagglutinated bovine erythrocytes whereas only one of the isolates agglutinated the equine erythrocytes. Polymerase chain reaction was used to amplify the F cleavage site gene of all isolates. Based on mean death time the Newcastle disease virus isolates could be placed in mesogenic or velogenic group.