Abstract: The effect of porosity on compressive strength of IMA carbon fiber reinforced M21 epoxy resin composite plate was studied by compression test, and a nonlinear relationship model between porosity and compressive strength was established. The results show that when the porosity no more than 1.87%, the compressive strength of the composite plate did not change obviously compared with that of the porosity of 0, and the effect of the porosity on the compressive strength was relatively small. When the porosity grater than 1.87%, the compressive strength of the composite plate was obviously lower than that of the porosity of 0, and the effect of the porosity on the compressive strength was relatively large. The compressive strength of the composite plate was predicted by the established nonlinear relationship model between porosity and compressive strength, and the prediction result was more accurate than that of Gürdal index model.