Asphaltene and wax deposition during production of many Iranian reservoirs has become a serious problem. In order to investigate the asphaltene precipitation behavior of various Iranian light and heavy oils and to predict their best method of prevention and/or curing the problem a comprehensive research study was launched. This article describes the precipitation potential of various reservoir oils, (8 samples, light and heavy) during their deposition. Three-phase equilibrium calculation of NGHIEM et al., is used. In the method pure asphaltene in the oil is considered as solid. The heavy part of the oil is split as precipitating and non-precipitating. The results of the study, which compares the onset and quantity of asphaltene precipitation of the oil samples revealed; a) the precipitation potential in the light oils depends strongly on the nature of asphaltene molecules in the oil which varies from one to another. b) Light oils behave in a more complicated fashion than heavy oils. c) The volatile oils were more unstable and the ratio of precipitation to the heavy fraction of the oil is highest. The paper describes the foregoing in detail.