In ion beam figuring process, typically, the smaller ion beam diameter has a good ability to “correct” the optical surface error, i.e. the smaller ion beam diameter indicates the higher material removal efficiency ε. The material removal efficiency is defined as the ratio of the volume of desired material removal to that of the real material removal. However the smaller ion beam diameter always results in more processing time, which usually decreases the process reliability. In this paper, the relationship between the material removal efficiency and the ion beam diameter is analyzed. The theoretical result shows that the material removal efficiency is a negative exponential function of the ratio of ion beam diameter to the spatial error wavelength, (i.e. d/λ). And when d/λ= 0.5, the material removal efficiency is 87%, which is acceptable in ion beam figuring process. When d/λ = 1, it rapidly decreases to 58%, which is unacceptable. According to theoretical analysis and simulation results, we recommend that d/λ should be less than 0.5 in order to obtain acceptable material removal efficiency in ion beam figuring process.