In this paper, the thickness/grain size ratio was proposed as an indicator for studying the size effect of micro deep drawing of a ball retainer. Experiments with AISI 304 stainless steel with 0.06mm thickness were performed by heat treatment and tensile test. Flow stresses at various thickness/grain size ratios were obtained. The commercial finite element software LS-DYNA was used to simulate the deep drawing process of the ball retainer. The simulation results agree well with experimental results. The effective stress on the lower fringe of ball hole decreases as the thickness/grain size ratio decrease. Effective strain and thickness shows less sensitivity with the thickness/grain size ratio according to the comparison. It determined that the thickness/grain size ratio would be an important indicator for process design as it corresponds to a heat treatment condition.