In dislocated Cz-Si crystals, rows of flow patterns (FP) and Secco etch pits (SEP) (2–3 mm in length, along <110> direction) can be revealed by Secco etch without agitation. In this study, the crystal defects forming FP-SEP rows in dislocated Cz-Si crystals are investigated by transmission electron microscopy. Microdefects, 0.1 μm in size, are observed in a row along a FP-SEP row, <110> direction. These defects were identified as oxygen precipitates with or without dislocation loops (interstitial-type), and voids with oxidized interiors. We conclude that FP originate from interstitial-type dislocation loops, and SEP are due to oxygen precipitates or voids.