Abstract: Gear oil sample (0.300 0 g) was taken, and 15 mL of HNO₃, 5 mL of H₂O₂ and 1 mL of HClO₄ were added and the mixture was treated in a microwave digester under programmed temperature raising. After cooling, the digested solution was evaporated to near dryness at 180℃. After cooling, the volume of the solution was made up to 20 mL with water. Pb, As, Cr and Cd in the solution were determined by ICP-MS. Internal standard method was used to match the matrix effect. Linear relationships between values of signal strength and mass concentration of Pb, As, Cr and Cd were obtained in the same range of 1.00-50.0 μg·L^-1, with detection limits (3S/N) of 0.05, 0.006, 0.04, 0.003 μg·L^-1 for the 4 elements respectively. Test for recovery was made by standard addition method, giving results in the range of 81.3%-96.4%, and RSDs (n=6) ranged from 1.2% to 4.3%.