The precipitate behavior in a floating zone silicon crystal produced from a Czochralski single-crystal ingot has been studied. Large precipitates of α-Si3N4 crystal, having a dimension of about 2 μm, were formed at the mid-depth in the wafer by means of annealing at a high temperature in an ambient N2 (70%) + O2 (30%) atmosphere. The precipitate number detected by cross-sectional X-ray topography increased with the increasing annealing time. Interstitial silicon is expected to eliminate the precipitate origins.