The oil identification of pay zone in very fresh waterbearing reservoir in China is hard problem to be solved by logging analyst. This is because the difference of resisitivity between oil and water zone in this kind of reservoir is relatively small. For this, logging analysts have been trying to find non-electric logging methods to solve the problem. Many methods have been found. The dynamic Poisson's ratio obtained from acoustic log is one of those. However, as we have known, the dynamic Poisson's ratio is useful for oil identification in high salinity- bearing sand. But, the mechanism of oil identification in very low salinity water- bearing sand is uncertain because interval transit time of the rock has a relation to the formation water salinity. In order to solve the above problem, an experimental study has been carried out with natural cores in Lab. under the reservoir conditions. The results of the experiment show that the Ave. values of dynamic Poisson's ratio are about 0.3 and 0.25 respectively for the cores saturated with fresh water and natural oil. Those also show that it is possible to identify the oil using dynamic Poisson's ratio even in very low salinity water- bearing sand.