Abstract: Rotating arc horizontal GMAW can solve the dripping of the molten pool in the horizontal welding.The rotating arc process not only can reduce the welding heat input by prolonging the welding path in the some welding distance caused by the arc rotation, but also disperse the arc force to affect the sidewall periodically resulting in supporting the fusion metal near the upper groove.The characteristics of joint formation in rotating arc horizontal GMAW was studied.The asymmetry of the microstructure in the joint due to the rotation of the arc was discovered.The reason and the mechanism of this phenomenon was analyzed and interpreted.