Abstract: The influences of welding parameters on porosity, hot cracking and spatter were investigated in partial penetration fiber laser welding, and the formation of weld defects was also discussed. The result indicates that porosity and hot cracking are less to form with increasing the welding speed. Porosity is the most significant as the defocus distance is 0, and the susceptivity of hot cracking increases with shifting the focus position from negative to positive defocus. Porosity results from the keyhole instability, whereas the keyhole instability results in the change of the solidification geometry, which increase the susceptivity of hot cracking. Spatter occurs in low speed fiber laser welding owing to the convex part of weld pool just in the front of keyhole, which is probably determined by the keyhole stability.