Abstract： The corrosion behavior of the hot-extruded SiCp/AZ91 （MMCs）, a magnesium alloy AZ91 based composite containing different amount SiC particles （i.e. 2%, 5%, 10% and 20% in volume fraction）, was investigated in 3.5%NaC1 solution saturated by Mg（OH）2 at 25 ~C by means of mass loss method, electrochemical measurements, SEM and TEM observations and XRD. The results indicated that： The corrosion rate of the SiCp/AZ91 MMCs was much greater than that of AZ91 magnesium alloy, and which increased as the increasing volume fraction of micron-sized SiC. The strengthening SiC particles exhibited little tendency with the alloy matrix to form galvan- ic couple, thereby little tendency to induce galvanic corrosion in micro-scale. But the addition of SiC particles resulted in a highly change in microstructure of SiCp/AZ91 MMCs, as consequence, the corrosion resistance of the composite became significant inferior, in comparison with those of the AZ91 alloy. There were two main reasons responsible for the greater corrosion rate of SiCJ AZ91 MMCs. Firstly, SiC oarticles oromoted ain refinement of B ohase, which increased the mi-cro-galvanic corrosion current density between fl phase and a phase. Secondly, the addition of SiC particles promoted the dynamic recrystallization nucleation of β phase at the interface SiC particles/ AZ91 matrix, which might be beneficial to the microgalvanic corrosion of the coupling of β phase and a phase there, therewith indirectly increased the corrosion rate of SiCp/AZ91 MMCs.