This paper deals with the detailed mechanism of lubricant applying effect and the influence of the depth of cut on lubricant applying effect. The experimental results indicated that the Vickers microhardness distribution in the zone to be cut markedly controlled reduction percentage in the cutting force. It is concluded that the lubricant applied to the pre-cut surface reduces friction at the microcrack openings on the pre-cut surface, which results in smaller shear deformation under high normal stress at the shear plane.