The flow pattern defects (FPDs) in as-grown and rapid thermal annealed <100> boron-doped CZSi wafers were investigated in this paper. The experimental results showed that a hole occurred on the apex of FPDs observed by atomic force microscope. The changes in the number and the size of the FPDs in as-grown and annealed wafers were measured by optical microscope after etching in Secco etchant. The size of FPDs became smaller, the density of FPDs reduced above 1100 o C annealing and became very low above 1200 o C annealing in an Ar atmosphere; the hole on the apex became shallower and larger. This mechanism was discussed.