Titanium alloys Ti6Al4V, as difficult-to-cut materials, have poor maChinability.Conventional cutting fluid serves both as a coolant and lubricant.In green drilling, water vapor as is recognized as an effective coolant, however, its lubrication properties are not well known in drilling.The focus of this study was to investigate the friction characteristics between chip and tool in green drilling Ti6Al4V, and it is compared to that in sliding and turning process.A friction evaluation model was developed based on the equivalent model of drilling, then to calculate the effective friction coefficient.Results indicated that the friction coefficient on the tool-chip interface was considerably reduced in drilling by using water vapor, so the drilling forces reduced, too, the friction coefficient decreased as velocity increased in drilling, which is different from the law of sliding tests, the friction coefficient increased when lx increased.