Abstract: Theoretical inference, calculation of measured parameters and correlation of partial derivative curves show that, as to common sandstone reservoirs, the influence of cementation factor (m) on water saturation is more severe than that of saturation index (n) and the difference gets more distinct as water saturation goes higher. Higher m values have greater influence while lower n values have greater influence. It is conclude) that m is a key parameter to the calculation of water saturation and highly accurate m value is necessary. When accurate m and n values cannot be obtained a group of data with lower m value and higher n value should be selected from the reference data sets to which the reservoir conditions, water-bearing properties and oil -bearing properties are much close.