Newcastle disease is highly infectious viral disease causing huge economic losses worldwide. These losses can be prevented by control of viral diseases. Medicinal plants have been traditionally used for treatment of different diseases since long. In this study the effect of extracts from Glycyrrhiza glabra leaves are investigated against Newcastle disease virus (NDV) by an in-vivo assay. Seven groups of nine-day-old embryonated chicken eggs were inoculated with various treatments of different plant extracts. All the groups except uninoculated negative control group were inoculated with velogenic NDV strain; five groups received different concentrations of the three extracts. Daily observe the rate of embryo survival. Allantoic fluid from treated eggs was collected for hem agglutination test. Results showed that embryo survival rate was higher 300µg/mL treated group as all the extracts showed antiviral activity. Similarly, the plant extracts effectively control virus as no viruses were identified in the allantoic fluids of all groups treated with low doses of plant. The current results have clearly verified that all the extracts especially that of methanol 300µg/mL from leaves of Glycyrrhiza glabra have strong antiviral activity against NDV in vivo.