Abstract: The formation mechanism of crack defects in resin---rich areas was studied for the quartz fiber reinforced polyarylacetylene(PAA) composite. The curing characteristic and shrinkage of PAA resin were evaluated by means of differential scanning calorimetry, infrared ray, three---point bending and micrograph. Moreover, the influences of mechanical performances of PAA resin, preform structure and fiber volume fraction on formation of crack defects were investigated. The results indicate that large curing shrinkage and brittleness of PAA resin are the main reasons for crack defects, and the crack defects can be reduced by increasing the fiber volume fraction and reducing the resin---rich area. Besides, resin modification by phenolic resin was proved to be an effective method to reduce crack defects.