Background: Finding alternatives to antibiotics for poultry production is very important because there are increasing concerns about antibiotic resistance. So, researchers have been directed to the research back to natural antimicrobial products. Some researchers stated that probiotics can stimulate the immune system and play an important role in shaping the immune system. Objectives: Theaimof this studywasto examine the effect of a commercial probiotic mixture (AquablendAvian®) supplementation to the drinking water of broiler chickens on the immune response against Newcastle and influenza diseases vaccines. Materials and Methods: In this study, 180 one-day-old broiler chickens were purchased and divided randomly into 3 groups (n = 60 for each group). Chickens in groups A and B received 300 mg of the probiotic in drinking water for first 3 days and first 7 days, respectively. Chickens in group C were kept as a control group and did not receive probiotic. All groups were vaccinated with live Newcastle vaccine (B1 strain) intraocularly on 8th day, and AI-ND killed vaccine (subtype H9N2) subcutaneously at the back of the neck on 8th day. Two mL of blood samples were collected before vaccination as well as on days 14, 28 and 35 postimmunization. Ten chickens of each group were bled randomly and an antibody titer against Newcastle disease vaccine and AI-ND killed vaccine(subtype H9N2) was determined by the hemagglutination-inhibition test. Results: The results of the present study showed that oral administration of the probiotic for 7 days significantly increased the specific antibody response to Newcastle vaccine compared to the control group (0.75 - 1.6 log, based on log2), while the probiotic administration had no significant effect on antibody productions against avian influenza vaccine as compared to the control group. Conclusions: Oral administration of Aquablend Avian® probiotic strains including Lactobacillus, Streptococcus and Bifidobacterium for 7 days can enhance the systemic antibody response to Newcastle vaccine in broiler chickens.