Abstract: As one of the critical indices for the measurement of port oil, the density of crude oil for custody and loading has been suffering from large measureing errors long since, resulting in considerable losses to national and enterprise benifits. To conquer this problem, Dalian Oil Port Office, in two years, carried out a series of tests, finding out that the errors are from both artificial and natural factors. The artificial errors are committed by the techincian during operation, whereas natural errors are created due to improper technique rather than personnel. With the nature of quick solidification, the crude oil of Daqing is characterized by high wax content, high pour point and high viscosity. At high heating temperature, the wax crystal in it will be thoroughly melten, and the crude oil will be more flowable and less viscous. As a result, the density of the oil measured at 40℃ is smaller than that measured at 37℃. At a temperature higher than 40℃, the reduction of measured density intends to be aconstent comparing with the standard density. though the optimal measuring temperatureranges from 40℃ to 41℃. Therefore, the optimal temperature shall be selected for density measurement as specified in national standard (GB/T 1884-92), so as to keep adequate oil flowability and uniform samples and to eliminate the artificial influence on measurement.