To study the stimulating mechanisms of immunostimulants in shrimp autogenous immunocompetence ,the immunostimulating effects ofβ2glucan ,lipopolysaccharide (LPS) ,inactivated Vibrio harveyi and Vibrio anguillarum on the hemocytes and phenoloxidase (PO) from shrimp , Penaeus chinensis , were investigated in this study. After shrimps stimulated with lipopolysaccharide (LPS) ,β2glucan ,inactivated Vibrio harveyi and inactivated Vibrio anguillarum ,the number of their overall hemocytes increased 83. 4 % ,52. 0 % , 73. 4 % and 111. 3 % respectively ,in which the number of semigranular cells increased 100. 4 % ,67. 3 % ,57. 2 % and 102. 9 % ,and the number of granular cells increased 47 % ,10 % ,127 % and 173 % ,respectively. And the amount of PO increased 104. 7 % ,81. 3 % 30.1%and40.4%,respectively,whereas the unit activity of PO remains almost the same before and after stimulation.Under the stimulation ofβ2glucan or LPS ,the amount of rough endoplasmic reticulum (RER) of semigranular cells and granular cells increased greatly and with swelled cristae. And the number of free ribosomes and mitochondria increased obviously and the number of secretary vesicles decreased greatly. However ,the ultrastructure of hyaline cells changed less before and after stimulation. Under the stimulation of inactivated Vibrio anguillarum ,the amount of RER in semigranular cells and granular cells increased greatly. And the number of free ribosomes increased obviously in granular and hyaline cells. The number of mitochondria increased obviously and its nucleoplasmic ratio decreased in hyaline cells. Under the stimulation of inactivated Vibrio harveyi ,the amount of RER and the number of mitochondria increased greatly and nucleoplasmic ratio decreased obviously in semigranular cells and granular cells. The amount of RER and the number of mitochondria increased obviously and nucleoplasmic ratio decreased in hyaline cells. Among the four kind of immunostimulants ,β2glucan and LPS acted mainly on semigranular cells and granular cells , whereas inactivated Vibrio harveyi and Vibrio anguillarum acted on semigranular cells , granular cells and also hyaline cells. Therefore , it can be concluded that 1) the enhancement of shrimp autogenous immunocompetence is finally realized via increase of OP production instead of PO unit activity ,2) the enhancing mechanisms of polysaccharides and inactivated vibros are different , 3 ) the enhancement of shrimp immunocompetence is finally realized via inducing RER hyperplasia in granular cells to increase PO production after stimulation byβglucan and LPS ,4) the enhancement of shrimp autogenous immunocompetence is finally realized via stimulating the proliferation of granular cells to increase PO production after stimulation by inactivated Vibrio harveyi and Vibrio anguillarum , and 5) the immunostimulating effect of polysaccharides is much better than that of inactivated vibrios.