The influence of Spodoptera litura nuclear polyhedrosis virus (SlNPV) on the phenol oxidase activity and haemolymph black of the Spodoptera litura (Fabriciu) larvae was observed by using SlNPV as the immune stimulating factor. The results indicated that the melanization rate in the hemolymph of the host larvae infected with SlNPV at the early 2nd instar began to decrease gradually from day 1 after the virus infection, the ability of the virus in inhibiting the melanization was enhanced with the passage of time after the virus infection and with the increase of host day age; within the 5-day observation time after infection, the percentage of melanization was consistently lower in virus-infection group than non-infected (healthy) group and this difference was significant on days 4 and 5 after infection; in virus infected group, the phenoloxidase activities in the larval body and in the hemolymph of S.litura larvae gradually rose up in the first three days after infection but then dropped down; the phenoloxidase activity in the larval body of S.litura larvae was consistently higher during the first three days but began to be lower from days 4 after infection in the virus-infected group than non-infected group, and this difference was significant on days 2,3 and 5; compared to that in non-infected group, the phenoloxidase activity in the hemolymph of the host larvae in virus infected group began to be enhanced from days 2 after infection but sharply dropped down on days 5 after infection. 