Newcastle disease (ND) is an extremely viral disease that has tremendous impacts on poultry production worldwide. Increasing and repeating ND outbreaks and suspecting of fifth pandemic occurrence demonstrated the need for novel medicines to control the disease. Chlorella vulgaris (CV) microalgae have recently emerged as natural alternatives with antiviral activity, in current study, ethanolic Chlorella vulgaris) ECV) extract was prepared and evaluated as an effective antioxidant and antiviral agent against Newcastle disease virus (NDV) in vitro and in vivo stages. The effect of ECV extract with three concentrations (50μL, 100μL, and 200μL) was estimated on embryonated chicken eggs (ECEs). The data revealed that 100 μL of ECV extract is a nontoxic dose, as evidenced by the absence of embryo deaths, and has effective antiviral activity by using a mixture of 0.2 mL of NDV with 108.5 EID50/ml with 0.2 mL of ECV extract, which led to decrease of viral titer to 103.4 EID50/ml with complete inhibition of NDV replication and loss of haemagglutination (HA) activity. Later on, in vivo study was applied in 28 days old Sasso chickens to evaluate the activity of ECV extract at 1 g/kg concentration according to invitro assessment as it is nontoxic effective antiviral dose in the drinking water before and after the NDV challenge. The used assessment parameters in this study were clinical signs, post-mortem (PM) lesions and histopathological pictures and it showed the effective role of ECV extract in viral replication inhibition in the treated groups when compared to control ones. Also, Real-time PCR was conducted to estimate NDV titer after challenge, in the group (III) and group (IV) showed a decrease in viral shedding at 3rd and 5th day post challenge (dpc) and a complete absence of viral titer at 7th dpc in the prophylactic group (III) in comparison with the positive control (II). These findings illustrated the potential role of ECV extract in overcoming NDV infection under field conditions and advised using it as an antiviral agent.