Abstract: This study is to investigate the wear mechanism develops during drilling of High Speed Steel of molybdenum type drill 5 mm in diameter coated TiCN, TiN and TiAlN on the medium carbon alloy steel work piece. The performance tests were conducted on CNC milling machine at spindle rotation of 1,600 rpm and feed rate of 20 mm/min. A carbon steel plate with a thickness of 30 mm was used as a work piece and the depth of drill was set at 25 mm. After drilling, the microstructural changes on each coated drill were observed under the scanning electron microscopy (SEM). In this paper the wear mechanism operated during drilling was discussed. Generally, wear mechanism operated during drilling were found to be a complex combination of abrasive, adhesive and thermal wear...