SUMMARY A study was conducted to investigate the effect of dietary supplementation of Chlorella (Chlorella vulgaris) to replace antibiotics in the diets of broiler chickens. A total of 600 1-d-old male broiler chicks were randomly assigned into 5 treatments with 4 replicate pens per treatment (30 birds/replicate pen) for 4 wk. A corn-soybean meal basal diet (control) was formulated, and 0.1% virginiamycin as antibiotic growth promoters (AGP), 1.0% dried Chlorella powder (DCP), 1.0% Chlorella growth factor (CGF), and 1.0% fresh liquid Chlorella (FLC) were added to the basal diet to form 5 dietary treatments. No significant differences were found among the treatments for feed intake or feed conversion of broiler chickens during the whole experimental period, but the BW gain was significantly higher (P < 0.05) in AGP and Chlorella-supplemented groups compared with the control group. The number of white blood cells was significantly higher (P < 0.05) in broilers fed FLC compared with DCP, and the number of lymphocytes was also significantly higher (P < 0.05) in the FLC treatment compared with AGP and DCP treatments; however, supplemental AGP and Chlorella had no effect on other blood leucocytes of broiler chickens. Dietary supplementation of Chlorella significantly (P < 0.05) increased the plasma IgA concentration of chickens compared with AGP and control. In addition, plasma IgM concentration was higher in DCP and FLC treatments than the control treatment, and plasma IgG concentration was also higher in the FLC treatment compared with other treatments. Supplemental AGP and various forms of Chlorella did not affect the Escherichia coli and Salmonella concentration in the intestinal microflora of broiler chickens, but the population of Lactobacillus was significantly increased (P < 0.05) when birds were fed FLC. It is concluded that dietary supplementation of FLC improves BW gain, immune characteristics, and the production of Lactobacillus bacteria in the intestinal microflora of broiler chickens.