We have investigated the formation behavior of grown-in defects in silicon crystals. The void defects are formed by the aggregation of the super-saturated vacancies during the growth process and the V/G parameter (V, growth rate; G, axial temperature gradient at the crystal-melt interface) controls the vacancy concentration. We investigated how the void defects were influenced by V/G and the cooling rate. The density of void defects (N v ) is proportional to the cooling rate (C r ) to the power of 1.5; that is, N v =βC r 1 . 5 . This relation holds up to a cooling rate of 40K/min. On the other hand, the coefficient β increases with a decrease in V/G. The relation of N v =βC r 1 . 5 does not hold for cooling rates >40K/min, and oxygen precipitation increases instead of void formation. This behavior supports the void formation model of Voronkov. We also discuss this phenomenon in this paper.