Abstract: Superplastic deformation was found in extruded SiCP-LY12 composite without any refining treatment. Maximum elongation above 150% was achieved at temperatures of 495-510 C and initial strain rate of 2×10-4s-1.Analysis of test data indicates that there is a transition of deforming mechanism near the optimum parameters,which leads to a significant increase in elongation to failure,while the value of strain rate sensitivity(m)of superplastic composites increases with elevating temperature or decreasing strain rate;however,the maximal value of m seems not corresponding to the highest value of elongation. A unique strain-hardening stage took place after the ordinary dynamic recrystallization stage and endured to failure. Microstructure observation found that dynamic recrystallization occurs in the initial stage of deformation,which hints that the unique strain hardening phenomenon might be related with recrystallized grain growth. Furthermore,interfaces between SiC particles and matrix were severely weakened during deformation and turned to be a preferential path of crack propagation。which is harmful to achieving high elongation.