The major element composition of Late Cenozoic basalts from Leiqiong area, South China is analyzed by wet chemical method. Coupled with some previous data, Fe3+/ΣFe ratios of these basalts can be divided into two groups, 0.20-0.35 (named as normal samples) and 0.45-0. 66 (named as abnormal samples). The oxygen fugacity in the basalt formation is calculated by the empirical equation which linked fo2 to Fe2O3/FeO. The relative fo2 of normal samples are about FMQ+1.1 to FMQ+2.4, slight higher than that of the mantle xenoliths found by previous researchers (FMQ-0.943 to FMQ + 1.235). The difference may be attributed to the calculation methods, but the reiative fo2 of abnormal samples are around FMQ+3.3 to FMQ+4.8, 2log units higher than that of their source region, and that difference can not be explained in the same way as above. Through discussing the effect of magmatic processes on fo2, it indicates fractional crystallization, degassing and oxygen exchange between ascending melts and crust material and lithospheric material can not result in so large fo2 and high Fe3+/ΣFe ratio in abnormal samples.
