The grown-in defects in CZ-Si crystals grown at the rate of 0.4 mm/min was investigated by bright field IR laser interferometer and TEM. We have revealed that the defect is composed of the stacking fault and perfect dislocations. The stacking fault was extrinsic Frank partial type and the ends of the perfect dislocations were contacting the edge of stacking fault. From the observation, we concluded that the stacking fault was formed by the agglomeration of self-interstitial atoms during crystal growth. The dislocations are considered to be generated from the edge of stacking fault. The cumulative interstitial concentration which contributes to the formation of stacking faults was estimated to be in the order of 10(13)/cm(3) from the size and volume density of the stacking faults.