A full-potential and all-electron first-principles method for calculation of positron annihilation characteristics in solids is presented, in which the wave functions of electrons and positron are expanded by linearized augmented plane waves and plane waves, respectively. The method is employed to calculate the coincidence Doppler broadening (CDB) of positron annihilation radiation spectra from Si bulk, monovacancy, divacancy, and various vacancy-oxygen complexes. In addition, experiments were performed on electron-irradiated Cz (Czoehralski grown) Si samples after post-irradiation annealing up to 600 degreesC. Though the longer lifetime for the defects is almost constant at about 300 ps during annealing, the CDB spectra for the Cz Si samples exhibit a distinct stage around 350 degreesC, indicating a marked change in the defect nature after annealing at 350 degreesC. By combining the experiments and theories, the observed annealing behavior is clarified to be due to the formation of V4O2 in the Cz Si.