In this paper, the micron SiCp/AZ91 composite fabricated by stir casting is forged at 420 degrees C with 50% reduction initially, and then subjected to extrusion at 370 degrees C with the ratio of 16. Microstructure evolution of magnesium alloy containing micron particles during room temperature tensile process has been investigated using transmission electron microscopy. Results illustrate that the micron particles lead to inhomogeneous deformation of magnesium matrix, thus causes dislocation pile-up near particles and the formation of twinning away from particles. Larger stress concentration occurs at the end of some micron particles, which results in the appearance of microcracks. In the course of subsequent deformation, the composite fractures due to rapid extension of microcracks. (C) 2012 Elsevier B.V. All rights reserved.