The leaching behavior of battery grade lithium cobalt oxide (LiCoO2) was studied in Na2SO3-H2SO4 aqueous solutions with low H2SO4 concentration. The effects of H2SO4 concentration, leaching time, leaching temperature, and dosage of Na2SO3 on the leaching rate of LiCoO2 were investigated. The experimental results show that all the cobalt in LiCoO2 can be dissolved in the following conditions: the H2SO4 concentration of 0.4 mol/L, the leaching time of 0.5 h, the leaching temperature of 70°C and the Na2SO3 dosage being equal to the weight of LiCoO2. It is proved that the leaching of LiCoO2 in the system of Na2SO3-H2SO4 solutions is much more effective than that in other leaching systems. Moreover, the consumption of H2SO4 can be lowered greatly. Based on the leaching of pure LiCoO2, the Na2SO3-H2SO4 leaching system is further applied in the treatment of spent Li-ion battery and the leaching rate of cobalt can be achieved as higher as more than 98%.