In the present study, the effects of Chlorella vulgaris on blood and immunological parameters of Caspian salmon (Salmo trutta caspius) before and after exposure to Viral Nervous Necrosis (VNN) virus were examined. In this regard, four treatments in triplicate were chosen. Groups included one control and 3treatments (T₁, T₂ and T₃).  Fish in control group, T₁, T₂ and T₃ were fed diets supplemented with 0, 1×10⁸, 2×10⁷ and 3×10⁶ chlorella/450 g of food respectively, for sixty days. In addition, a virus supernatant was prepared from infected wild golden grey mullet (Liza auratus) and used for virus challenge of S. trutta caspius. Virus was injected intraperitoneally and blood samples were collected before and 14 days after the challenge. Immunological (IgM, C_3, C₄, total protein, respiratory burst, albumin and lysozyme) and changes in blood parameters (RBC, WBC, Htc, Hb, MCH, MCHC and MCV) were also measured. Results showed that C. vulgaris could act as a natural immunestimulant. Also, the alteration trend in hematological and immunological parameters showed that experimental fish could be considered to be resistant to VNN virus after exposure and fish treated with C. vulgaris were more resistant in comparison to those in the control group. The dose used in T₁ (1×10⁸ chlorella/450 g food) was the most effective approach with significant differences.