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We have studied the influence of C and Si ion implantation with different implantation doses on yellow luminescence (YL) from GaN. Three kinds of GaN samples were used. In their as-grown states, #1 samples had strong YL, #2 samples had no YL, while #3 samples had weak YL. Our experimental results show that: (i) after annealing at 950 degreesC, the YL intensity for Si ion implanted #1 sample decreased with increasing implantation dose, while that for C ion implanted #1 sample exhibited a reverse rule; (ii) for #2 samples, C ion implantation produced much stronger YL than Si ion implantation did after annealing at 950 degreesC; (iii) the YL intensity sequence for Si ion implanted and 950 degreesC annealed #1, #2, and #3 samples was consistent with that for the unimplanted #1, #2, and #3 samples. However, the YL intensity sequence for the C ion implanted and 950 degreesC annealed #1, #2, and #3 samples reversed with that for the unimplanted ones. In order to explain all these phenomena, we suggested a physical model which claims that the deep C center is another important origin of YL in GaN, and during C ion implantation, the C ion prefers to combine with a V-Ga to form a C-Ga.