TiCN and TiAlN coated twist drills were subjected to drilling tests to investigate the failure mechanisms during drilling operation. The drilling tests were performed on a carbon steel plate with a thickness of 25 mm and the depth of drill was set at 20 mm. The drill performance parameters were set at a spindle rotation of 1,600 rpm and feed rate of 20 mm/min. Each sample was then subjected to Scanning Electron Microscopy examination to investigate the wear mechanisms operated during drilling. Microstructural examination showed that the abrasion, adhesion and thermal wear mechanisms are operated during drilling process.