Polybenzimidazole (PBI) is one of the engineering plastic materials. This material is expected to increase demand due to its excellent strength properties and heat resistance. It was reported that high-speed end milling of this material using the coated diamond tool was effective. However, it is a problem that the micro-crack remains in the finished surface after cutting. In this study, the damage mechanism of finished surface in high-speed milling of PBI material was investigated. As a result of the observation in the finished surface, it is clarified that many cracks perpendicularly occurs for the cutting direction, and these cracks affect the surface roughness. Also, it is clarified that these cracks propagate from the surface to the inside of workpiece material and the direction of crack propagation is different by the size of the rake angle. The progress direction of the crack tends to approach the cutting direction with the increase of the rake angle. Therefore, the surface damage expands using the large rake angle, when the finishing edge passed above the cracks.