Abstract: Two kinds of unloading methods, vacuum — and pressure — operated methods, can be used to eliminate the phenomena of vaporation which are likely to occur due to the high temperature in summer during unloading light oil tanker by means of the currently used top — unloading process. The comparative study on these two methods, made as a special subject by Petro — chemical Sales Company based on site investigation and the analysis on measured data, shows that the pressure — operated unloading is superior to vacuum — operated unloading, and is wothy of recommendation. Using compressed air to raise the residual pressure at the suction side of pump, this method has the following advantages: no air-lock or cavitation, less evaporation loss, shorter unloading time, lower energy .consumption, safer operation condition and better environmental protection. Pressure — operated unloading can not only avoid the problem encountered during light oil unloading in summer, but also can yield considerable economical and social benifits. According to the appraisal by China Petro — chemical Corporation (Sinopec), this method is considered wothy of populization for application both theoretically and technically. Also, this technical achievement was awarded the Second Class Prize of Scientific and Technical Progress in Sinopec.