Based on modification of the simplified coherent potential approximation,a model for the band-gap energy of Inx GayAl1-x-yN is developed.The parameters of the model are obtained by fitting the experimental band-gap energy of their teary alloys.It is found that the results agree with the experimental values better than those reported by others,and that the band-gap reduction of InxGayAl1-x-yN with increasing In or Ga content is mainly due to enhanced intraband coupling within the conduction band,and separately within the valence band.