Abstract: A hydroponic experiment was conducted to determine the effects of different levels of potassium (0, 4.0 mmol·L^-1, 8.0 mmol·L^-1 and 16.0 mmol·L^-1) on the physiological indices of different tomato cultivars nematode-susceptible (HS), nematode- resistant (HR) and common (CK) cultivars. Based on the results, the activities of SOD, POD and CAT, and the contents of total phenol and flavonoid increased significantly with increased potassium concentration within 0~8.0 mmol·L^-1. On the other hand, MDA content decreased significantly with increased potassium concentration. However, the activities of SOD, POD and CAT, and flavonoid content dropped at potassium level of 16.0 mmol·L^-1 for HR cultivar. Contrarily, MDA content increased if potassium level was 16.0 mmol·L^-1 for HR cultivar. The activities of SOD, POD and CAT, along with the contents of total phenol and flavonoid for HR cultivar were higher and MDA content lower than for HS and CK cultivars. Test of significance of the degree of interaction between potassium and cultivar showed that the activities of protective enzymes under potassium application were higher for HR than for HS and CK cultivars.