Green cutting is one of the developing tends in the industry field. Water vapor can be introduced in metal cutting as coolant and lubricant due to its pollution-free, generating easily and unneeded disposal. Therefore, a special generating system is developed to produce suitable water vapor, and a simulation to the velocity of water vapor jet flow is presented. Then tool wear was investigated and a new capillary model is proposed, based on the experimental results. According to the boundary-layer theory, the kinetics equations of flow were solute. The velocity and flux of molecule are presented. In the capillary, the adsorption of tool-chip interface results in boundary lubricating film; the conical shape of capillary limits the depth of coolant and lubricant penetrating; and the negative press is the motility for coolant and lubricant penetrating. The study results show water vapor can decrease tool wear about 10% times and 20% comparing to cutting fluids and dry cutting, and water vapor could be a potential solution of green cutting.