Abstarct: The hemocytes are one of the important components of immune system of insects against various stresses such as pathogens attack, parasitoids, starvation periods and thermal changes. Hemocytes characteristics recognition, their frequency and the investigation of the amount of Phenoloxidase enzyme activity is important in the cellular immunological studies. In this study hemocytes of fourth instar larvae of potato tuber moth Phthorimaea operculella (Zeller) were identified after staining with Giemsa and by light microscopy at 40x magnification. 5 types of hemocytes include of prohemocytes (PRs), plasmatocytes (PLs), granulocytes (GRs), oenocytoids (OEs) and spherulocytes (SPs) were identified. The frequency of hemocytes and the activity of Phenoloxidase enzyme in the fourth instar larvae of this pest were investigated under the influence of thermal stresses and also after feeding on five potato cultivars (Sante, Agria, Fontane, Alpha and Alpha paboland). The immune responses of fourth instars larva of the potato tuber moth was also investigated against two isolates of entomopathogenic fungi Beauveria bassiana (Fashand and 47) After 3, 6 and 10 hours. The results of different thermal effects on immune system of potato tuber moth showed that total hemocyte count (THC) and PLs of larvae which were for 24 hours under temperature stress of 35 ° C, was increased significantly compared to the control (25±1 °C). Also, chill stress (4 °C) showed a significant decrease in THC, PLs and OEs compared to the control. The effect results of two isolates of fungi on cellular defense of potato tuber moth, showed that the THC, PLS and GRs increased significantly after 3 hours compared to the control. Increase in the number of homocysts in the isolate of Fashand was clearly seen. Then the THC, PLs and GRs decreased gradually at 6 and 10 hours after injection. Prohemocytes as stem cells, showed a significant decrease in 6 hours after spores injection. The results effects of five potato cultivars on on immune system of potato tuber moth showed that the maximum THC, GRs and OEs were attributed to the larvae fed on Agria cultivar and the minimum frequency of these hemocytes were in the larvae fed on Alpha cultivar. The maximum PRs and PLs were obtained from larvae fed on the Sante and Agria cultivars, respectively. The amount of enzyme activity of phenoloxidase was measured in the presence of L-DOPA substrate. This parameter was varied in larvae which were under the influence of thermal stresses and also larvae which fed on different cultivar. The temperature of 25±1 °C resulted in a significant increase in the activity of phenoloxidase enzyme and temperature of 35 ° C resulted in a significant decrease in this parameter. The maximum activity of Phenoloxidase enzyme was in larvae that fed on Agria cultivar and minimum of it was attributed to the larvae fed on Alpha paboland cultivar. Due to low immunity of this insect at low temperature, the activation of the immune response against both Fashand and 47 isolates, as well as the Agria cultivar, therefore, the use of a cooling system in the storages, combining the use of resistant cultivars and entomopathogenic fungi will help to better control of this.