Abstract: The weld cross-section formation in oscillating arc narrow gap vertical GMAW was investigated and the effect of oscillating parameters on the wend cross-section formation was analyzed. The results show that the weld cross- section formation was characterized with convex surface during oscillating arc vertical-up GMAW, while it was concave during vertical-down GMAW. The increase in oscillating angle and oscillating speed was beneficial to the increase of weld width and reduction of convex height in oscillating arc narrow gap vertical-up GMAW, while the dwell time had little influence on the weld cross-section formation. The heat input per unit length in oscillating arc welding was divided into two parts which were input heat per unit length in oscillating process and in dwell process, respectively. The input heat per unit length in oscillating process could affect the convex height of weld cross-section, while that in dwell process could affect the weld width. The oscillating parameters controlled the weld cross-section formation by affecting the distribution of input heat in oscillating arc narrow gap vertical-up GMAW.