This paper presents an experimental study on the dimensional error in ball-end milling. In the 3D free-formed surface machining using ball-end milling, while machining conditions are varied due to the Z component of the feed and existing hemisphere part of the ball-end mill, the mechanics of ball-end milling are complicated. In the finishing, most of cutting is performed the ball part of the cutter and the machined surface are required the high quality. But the dimensional errors in the ball-end milling are inevitably caused by tool deflection, tool wear, thermal effect and machine tool errors and so on. Among these factors, the most significant one of dimensional error is usually known as tool deflection. Tool deflection is related to the instantaneous horizontal cutting force and varied the finishing cutting path. It lead to decrease cutting area, thus resulting cutting forces but the dimensional precision surface could not be obtained. So the machining experiments are conducted fur dimensional error investigation and these results may be used for decrease dimensional errors in practice.