Background:Nowadays natural products such as pure compounds and plant extract scan provide unlimited opportunities for new antiviral drugs. Newcastle disease virus (NDV) is one of the most important viral diseases in poultry industry. Vaccination could provide protection against NDV outbreaks, but it is not sufficient because infections by NDVs have remained frequent around the world.Objectives:The current research aimed to study Achillea millefolium and Thymus vulgaris antiviral activity against Newcastle disease virus (NDV).Materials and Methods:The antiviral activity of the plants was measured by the reduction assay of viral titer, and explained by inhibition percentage (IP).Results:Inhibition percentage was determined as 10 1.75, which indicated the ability of the extracts to reduce the viral potency by more than 56 folds.Conclusions:Both plants were found effective against Newcastle disease virus.