Flow pattern defects (FPDs) is one kind of grown-in defects in large diameter Czochralski silicon (Cz-Si) crystals. The evolution of FPDs in lightly doped Cz-Si crystals during secco etchant (50% HF: 0.15 mol L-1 K2Cr2O7=2:1) etching process, was studied in detail firstly. The results also showed that the outline of FPDs became larger and the voidon the tip of FPDs changed into a shallow hole with the increasing of etching time. A parabola model was firstly put forward to explain the evolution of FPDs in Cz-Si wafers during the procedure. Furthermore, the microstructure of FPDs was observed by optical microscopy andatomic force microscopy, the results showed that the outline of FPDs was parabola with several steps and two heaves were firstly foundon the left and right sides of the voidon the tip of FPDs. All the results provide forceful evidence to that FPDs is one kind of void-type as-grown defects. These are very useful to investigate FPDs in Cz-Si wafers further and explain the annihilation of FPDs during high temperature annealing processes.