The principle and method of measurement and calculation of the total volume fraction of voids of RF sputtered amorphous silicon by speetroscopic ellipsometry and effective medium theory is described. It is found that the changes in the electrical and optical properties among amorphous silicon films deposited in different argon pressure may be satisfactorily explained in terms of changes in the total volume fraction of voids. This suggests that it is significant to measure and calculate the total volume fraction of voids of a-Si. The good agreement between the result of total volume fraction of voids and the measured result of total volume fraction occupied by Ar atoms shows the validity of the method.