The tensile crack initiation and propagation of M50 bearing steel were investigated. It is found that the primary carbides were fractured during the tensile test by in situ SEM, while decohesion of the carbide/matrix interface was hardly observed. In order to capture the initial tensile crack, quasi-in situ X-ray microtomography was performed, indicating that the earliest crack was formed from the broken interior carbides. This indicates that multiple cracks already existed in the matrix when they were observed on the sample surface. Electron backscattered diffraction (EBSD) and the finite element method (FEM) were applied to analyze the fracture behavior of carbide particles. The fracture of carbide particle is produced by the incompatible deformation between the matrix and carbides. The frequency of carbide fracture was found to be related with its fracture strength, size and shape. M2C carbide fracture was more easily broken than the MC carbide during tensile test. In addition, the long axis of most M2C carbides is approximately parallel to its (0001) plane and normal to tensile stress, leading to cracking more easily. Finally, a suggestion of adding pre-deformation is proposed to make M2C carbides broken sufficiently and further refine the primary carbides. •X-ray microtomography is applied to analyze the evolution of primary carbides.•Incompatible deformation between the matrix and particles leads to carbide fracture.•M2C carbide is more easily broken than MC carbide.•Pre-deformation before diffusion annealing should be applied to refine carbides.