In this experiment, nitrogen ions were implanted into CZ-silicon wafer at 100 keV at room temperature with the fluence of 5 x 10(15) N-2(+)/cm(2), followed by rapid thermal processing (RTP) at different temperatures. The single detector Doppler broadening and coincidence Doppler broadening measurements on slow positron beam were carried out to characterize the defects in the as-implanted silicon and RTP-treated samples. It is found that both nitrogen-vacancy complexes (N-Vsi) and oxygen-vacancy complexes (O-Vsi) produced by nitrogen implantation diffuse back to the sample surface upon annealing. But the N-Vsi and the O-Vsi complete with each other and give a summed effect on positron annihilation characteristics. It is shown that the N-Vsi win out the O-Vsi in as-implanted sample and by RTP at 650 degrees C, 750 degrees C, which make the S-parameter increase; O-Vsi plays a dominant role after annealing above 850 degrees C, which makes the S parameter decrease. ((C) 2007 Elsevier B.V. All rights reserved.