Background and objective: Vitamin C is considered as the indicator of process severity, because of its high sensitivity to heat. There is an inverse relationship between vitamin C retention and the process severity. The aim of this study was to determine the effect of different common cooking methods on process severity and decrease amount of vitamin C in some vegetables. Materials and methods: In this study, potatoes were tested as boiled (with peel, without peel, sliced), shallow and deep fried, and cooking under pressure (with peel, without peel, sliced). Onions were tested as boiled (whole and sliced) and microwave (whole and sliced). Leafy vegetables: Leek, leaf of beet, parsley, coriander, common dill and the mixed of them were tested as boiled fried. All tests performed as triplicate and the contents of vitamin C retention were determined by colorimetric method according to the instruction number 5609 of the Institute of Standards and Industrial Research of Iran and were compared to the contents of vitamin C in raw vegetables. Results: The highest decrease of vitamin C was observed in boiled, sliced (without peel) and shallow fried potatoes and onions. In leafy vegetables highest loss of vitamin C was observed from boiled fried. The loss of vitamin C in potatoes and onions by using microwave were small and their vitamin C contents were similar to raw vegetables. Conclusion: The maximum retention of vitamin C in potatoes and onions are obtained by microwave method, with peel and whole, respectively.