Three different experiments were conducted to study the effect of dietary astaxanthin on the immune response, resistance to white spot syndrome virus (WSSV) and transcription of antioxidant enzyme genes in Pacific white shrimp Litopenaeus vannamei. Each experiment included one triplicate treatment group (fed with 80 mg/kg astaxanthin supplemented diet) and one triplicate control group (fed with basal diet), and the feeding period lasted for 4 weeks. The experiment of immune responses showed that dietary astaxanthin significantly affects the hemolymph immunological index, including total haemocyte counts, phagocytic activity of haemocyte, serum anti-superoxide radical activity, serum phenoloxidase activity, serum anti-bacterial activity and serum bacteriolytic activity. In the experiment of resistance, the shrimp was challenged with WSSV after the feeding stage was finished. The result showed that the mortality of the experimental group 11 days post infection was 76.3%, while that of the control group was 100%. WSSV infection result via polymerase chain reaction (PCR) detection method showed that 95.6% of the dead shrimp in the experimental group were PCR positive, whilst 71.4% of the survived shrimps were WSSV positive. At the end of the experiment, 90% of the shrimps in the experimental group were WSSV positive, while all the the control group were WSSV positive. The result indicated that dietary astaxanthin could increase the resistance to WSSV. In the experiment of transcription of antioxidant enzyme (cyt-MnSOD, CAT and GPX) genes, the effect of astaxanthin on the transcription of these enzyme genes in the hepatopancreas was studied using semi-quantitative RT-PCR analysis. The results suggested that astaxanthin could promote the antioxidant enzyme gene mRNA expression in the hepatopancreas of L. vannamei.