The positron lifetimes spectra of hydrogen floatingzone silicon crystal isochronally annealed at different temperatures and other three kinds of silicon crystals have been measured. It is found that after annealed at a temperature above 550℃ the new micro defects appeared. The defect concentration of the silicon crystal grown in hydrogen atomosphere is higher than that of other silicon crystals. It can be seen from the experimental results that the temperature at which the Si-H bonds begin to dissociate is about 550℃.