When the removal depth in micro milling is downsized, the effect of the crystal grain in material becomes relatively large. The crystal grain in materials, therefore, should be downsized to achieve high qualities and high reliabilities of the micro devices. The study discusses the effect of the crystal grain size on the surface finish, burr formation and the affected layer in micro milling of stainless steel. The crystal grains are reduced to an average size of 1.5μm by repetition of material forming and phase transformation. The milling tests were performed to measure the cutting forces, the surface finishes, the burr shapes and the thicknesses of the affected layers. The force component ratio of the fine grained steel is higher than that of the standard steel. The shearing force decreases in cutting of the fine grained steel; meanwhile, the friction and/or the indentation forces increase. Burr formation is reduced when the crystal grain size is small. In cutting of the standard steel, X component in the cutting force suddenly drops near the end of the groove and a large burr is formed on the edge of the groove. The affected layer largely depends on the crystal grain size. The thickness of the affected layer remarkable decreases in milling of the fine grained steel.