In this study, flank wear on CBN and PCBN tools due to cutting forces were studied. Turning tests were carried using cutting speeds of 100, 125, 150, 175 and 200 m/min with feed rates of 0.10, 0.20 and 0.30 mm/rev and constant depth of cut. The performances of tools were evaluated based on the flank wear and cutting forces. There is clear relationship between flank wear and cutting forces while turning hard martensitic stainless steel by CBN and PCBN tools. Low cutting forces leads to low flank wear formation and low cutting forces provided good dimensional accuracy of the work material including low surface roughness. Flank wear formation was mostly caused by abrasion and adhesion. The built up edges formed reduced the cutting forces and also causes the heat generated at tool tip and work interface. High cutting forces are identified and this may be due to heat and flank wear combinations. Flank and crater wear on the rake face and hard metal deposition due to diffusion of metals on the cutting tool surface are the damages occurred during process.