The information on the weathering of spilled oil is necessary to the study of the behavior and fate of the spilled oil, in which the information of the oil fraction content is important especially. In this study, a lab scale experiment was conducted and the effects of short-term weathering on the several crude oils (light, medium and heavy oil) were investigated using GC-MS. The concentrations of n-alkanes at various degrees of weathering were obtained, and the correlation between evaporation percentage and the concentration of n-alkanes was indicated. The effects of weathering on the crude oils and faction contents varied from oil to oil. The BZ28-1 crude oil was seriously weathered with the evaporation percentage of approximately 22% in 24 hours. The next is the CQH crude oil, and the SZ36-1 crude oil is weathered least with the evaporation percentage less than1%. The concentrations of n-alkanes fraction in the crude oils were approximately 70.2810-3, 38.1910-3 and 0.4210-3, respectively. The abundance of n-alkanes gradually decreased with the weathering time extended, and the degrees of concentration reduction were different among different carbon numbers. The concentration changes of the C13~C26 in CQH crude oil and the evaporation percentages were confirmed as the linear correlation, which could be considered as the indicated index of weathering degrees.