The deformation and fracture behavior of as-solutionized SiCw/AZ91 magnesium matrix composite was observed during the in-situ tensile straining in a transmission electron microscope. The results indicated that the dislocation slip was impeded by the SiC whiskers, a large number of dislocation pile-ups occurred around the whiskers. Microcracks nucleated predominately at the SiC whisker rich regions and the matrix near SiCw-AZ91 interfaces. The SiC whiskers and twins prohibited the crack propagation. The microcracks linked with each other and connected with the main crack, leading to the catastrophic failure of the composite. The fracture processes of the composite were found to be matrix-controlled. The poor ductility of the composite was attributed to the low inherent ductility of the magnesium alloy matrix and the local stress concentration because of the addition of the SiC whiskers. (C) 2003 Kluwer Academic Publishers.