This paper presents an experimental study of the application of acoustic emission (AE) signal for tool wear monitoring in the milling of Ti-6Al-4V alloy. Experiments were conducted and the corresponding AE signals were captured under different tool wear status. Initial analysis reveals that the AE signal contains useful information about the mechanism of the tool wear and can reflect the changing of the cutting parameters as well which show that the AE signal can be used as a reliable means for accurate tool wear monitoring. The comparison with other kinds of sensor signals also shows that the AE signal is more suitable for online tool wear monitoring in industrial environment. Based on these conclusions, AE signal can be used as a reliable signal for accurate tool wear monitoring.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.