Abstract: The experiments were carried out to investigate the optimum immune age and immune dosage of GC₀₂ coccidian vaccine. Ninety healthy cross broilers were chosen and randomly divided intoⅠ,Ⅱ,Ⅲ,Ⅳ,Ⅴ,Ⅵ groups(n=15) to study the immune effect of different immune dosage at the same immune times. Each chicken in Ⅰ-Ⅳ groups were orally inoculated with 4 000 ,8 000,16 000,20 000 sporulated oocysts of GC₀₂ coccidian vaccine, respectively . Chickens in Ⅴ group were not infected with sporulated oocysts and chickens in Ⅵ group were not inoculated. On the 7th day after the second immunity,Ⅰ-Ⅳgroups were orally challenged with 1.0×10⁵ sporulated oocysts for each chicken. On the 8th day after challenging with the sporulated oocysts, all chicken were slaughtered. According to lesion scoring and faces scoring, OPG, ACI, relative growth rate, feed conversion, etc., the effects of different immune dosage of GC₀₂ coccidian vaccine were evaluated. The results suggested that optimum immunization method was inoculating 8 000 sporulated oocysts per bird two times.Onehundred and eighty healthy chickens were randomly divided into A,B,C,D,E,F,G₁,G₂,G₃ groups (n=20) to study the immune effects of different immune age with the same immune dosage. Chickens in A,B,C groups were immunized with GC₀₂ coccidian vaccine at the different immune age; D,E,F and G₁,G₂,G₃ were positive and negative control groups respectively, the immune and challenge is the same as experiment one. The results concluded that GC₀₂ coccidian vaccine is safe and effectual in preventing coccidiosis, immunize with 8 000 sporalated oocysts per bird at the age of 7 days and 14 days can obtain the ideal effects.