An adjustment device of a grinding machine contains: a grinding unit and a tool fixing unit. The grinding unit includes a first guide rail and a second guide rail. The tool fixing unit includes a horizontal rotation mechanism, a locating portion, a third guide rail, a fourth guide rail, and a fifth guide rail. The locating portion is cylindrical, parallel to a second XY plane, and has a rotation axis defined on a center of the locating portion, such that the locating portion rotates along the rotation axis and a tool is positioned on the locating portion to rotatably grind the workpiece at different angles.