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 1100degreesC annealing and became very low above 1200degreesC annealing in an Ar atmosphere; the hole on the apex became shallower and larger. This mechanism was discussed. (C) 2003 Elsevier B.V. All rights reserved.