The results of pathological research in haemolymph on WSS of Litopenaeus vannamei are as follows： pathological changes in WSS are evident and similar among diseased shrimps by different infecting manner. First, total haemocyte count in diseased shrimps is lower heavily than that in healthy shrimps. The percentage of the hyaline cell in diseased shrimps is lower heavily than in healthy shrimps. The percentages of small and large granule haemocytes of diseased shrimps are all higher heavily than those in healthy shrimps. Second, the main micro-pathological changes of haemocyte showed that there was an abnormal distribution of hemocytes in haemolymph smear from diseased shrimps. Broken and disintegrated cells can often be seen in the haemolymph smear. Third, the ultla-micro-pathological changes showed that there were necrotic changes in every kind of haemocytes, and there were atypical apoptosis phenomena in certain hemocytes; There were also many WSSV in certain hemocyte nucleus. All above showed that hemocyte cell was the main target cell of WSSV.