Internal gettering (IG) behavior for Ni contamination in Czochralski silicon wafers was studied. The wafers were initially contaminated with Ni, and then isothermally annealed between 800 and 1000 degrees C for up to 16 h. The density of Ni-silicides at the polished surfaces and the density of oxide precipitates at the cleaved surfaces were obtained by the preferential etching method with Wright etchant. It was confirmed that (i) with an increase in annealing time, Ni-silicide density decreased and became less than the detection limit, and (ii) oxide precipitates were not detected in some of the wafers, in which Ni-silicides were not detected. The density of oxide precipitates with their size less than the detection limit was obtained after additional annealing at 1000 degrees C for 16 h, and the size of precipitates was obtained by calculations with assuming diffusion-limited growth. The critical size of oxide precipitates for the IG effect was defined as the size above which the Ni-silicides were not detected. It was concluded that (ii the critical size decreased with an increase in precipitate density and (ii) the critical size became less than the detection limit of approximately 200 nm by the etching method, when the precipitate density was higher than about 10(8)/cm(3)