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We have studied the effects of Si ion implantation and post-annealing on yellow luminescence (YL) from GaN. Two types of GaN samples grown by the metal-organic chemical vapor deposition method and labeled as GaN1 and GaN2 were studied. The PL spectrum of the as-grown GaN1 sample was dominated by a strong YL and that of the as-grown GaN2 sample was almost free from YL. After Si ion implantation with doses of both 1.3 x 10(13) and 1.0 x 10(16)cm(-2), the intensity ratios of YL to near band edge (NBE) emission (I-Y/I-NBE) for the GaN1 samples decreased markedly compared with those of the corresponding unimplanted ones both before and after post-annealing at temperatures up to 950degreesC. However, for the Si-ion-implanted GaN2 sample with a dose of 1.3 x 10(13)cm(-2), I-Y/I-NBE increased compared with that of the as-grown one both before and after post-annealing. Besides, the I-Y/I-NBE for Si-ion-implanted GaN1 with a dose of 1.3 x 10(13) cm(-2) increased monotonically with annealing temperature. Our results show that only the Si ion implantation being accompanied with high-temperature post-annealing could produce YL. The possible reasons for the marked reduction in I-Y/I-NBE for the GaN1 sample after Si-ion-implantation have been discussed. (C) 2002 Elsevier Science B.V. All rights reserved.