Abstract: An empirical formula was established to calculate the average thickness of oxide film of micro aluminum powder. The morphology, mass fraction of active Al, particle size distribution and composition of 21 samples of micro aluminum falling within two series were investigated using the measures of SEM, gas volumetric method, laser particle size analyzer and mass spectrum, respectively. The relationship between the average diameter and the oxide film thickness was discussed. The results show that the oxide film thickness is an exponential function to the average diameter of micro aluminum powder. There is an upper limit value of thickness which is associated with the purity of aluminum powder.