The behavior of Si in GaN has been investigated in the wide doping range from 1 x 10(17) to 9 x 10(19) cm(-3) by photoluminescence (PL). PL studies at 77 K for doped specimens show that the intensity of the near-band-edge emission increased monotonically as the Si concentration increases. The intensity of the yellow luminescence (YL), however, remains almost constant, which indicates that Si does not strongly influence the density of deep centers in GaN. The red-shift of the near-band-edge emission due to increase the Si concentration shows that bandgap renormalization (BGR) effect is much more important than the Burstein-Moss (B-M) effect in Si-doped GaN. With Si doping as high as 9 X 10(19) cm(-3), a new peak due to donor-to-acceptor recombination appears in the PL spectrum. This donor-to -acceptor emission peak can also be observed in the PL spectrum of Si-doped GaN after thermal annealing. We attributed the appearance of the new acceptor level to the formation Of Si-N. A model has been proposed to illustrate the Si behavior in GaN in a wide range of Si concentrations.