The lifetime spectrum and the Doppler broadening lineshape S parameter of positron annihilation have been measured for hydrogen float-zoned silicon single crystals which are isochronal annealed at seven different temperatures between 500℃ and 700℃. It appears that no changes have been found for the different annealed temperatures. The results are in agreement with that given in several references. The average intrinsic lifetime in the specimen is 0.221~0.004ns. In this paper the defect induced by hydrogen is discussed based on our annihilation data.