Acoustic Emission (AE) is the emission of elastic stress waves resulting from the deformation and fracture of materials. From viewpoints such as the maintenance of machining accuracy and the fail-safe, monitoring of cutting states is very important. By measuring AE signals generated during turning process, the state estimation of machining becomes possible. This report discusses about correlation between the AE signals and the phenomena on turning process. The results indicate that there is a good negative correlation between the shear angle and the AE signal level. The measurement of the AE total energy will permit the estimation of tool wear. When TiAIN coated tool or the workpiece of different hardness was used, the similar results were obtained.