Abstract: Using the representative hydrocracking tail oil as raw material，the effects of raw material composition and corresponding base oil composition, the range of the feedstock distillation range and the processing technology on the evaporation loss of the base oil were investigated by two evaporation loss analysis methods, NB/SH/T 0059 and SH/T 0695. It is found that the structural and composition of base oil were the key and the internal factors affecting the evaporation loss, and the content of alkanes in base oil could be increased by various technological means. The viscosity and distillation range of the base oil were the external factors affecting the evaporation loss，which could be reduced by optimizing the feedstock and operating the fractionating column. The effective measures to reduce the evaporation loss of base oil were put forward.

Key words: lube base oil, evaporation loss, composition