Script MC carbides are easily formed in the welding of Ni-base superalloys. In this study, the effect of script MC carbides on the tensile behavior of laserwelded DZ125L/IN718 joints in the fusion zone was investigated. The phase size, phase distribution and element distribution of script MC carbides, and the orientation relationship between script MC carbides and gamma matrix were carefully analyzed using SEM, EDS, TEM and HRTEM. The breaking behavior of script MC carbides was investigated through interrupted tensile testing at 650 degrees C. The results demonstrate that the script MC carbides are distributed in the interdendrites with an average size of 3.5 +/- 1.6 mu m. They are (Ta, Nb, W, Ti and Mo)C and incoherent with the c matrix. The fine script MC carbides are beneficial for improving both the strength and ductility of the fusion zone by improving coordinated deformation with gamma matrix. However, the script MC carbides are found broken very early. About 9% script MC carbides have been broken after welding. The number of broken carbides increases with the increasing plastic strain. The majority of the cracks are found at the necks of the script MC carbides. The breaking mechanism of script MC carbides reveals that the high breaking tendency of the neck is attributed to the relatively long arm length and, more importantly, to the minimum section width size. Modifying the carbide morphology into a sphere is suggested for lowering the breaking tendency of MC carbides.