  We present a novel model to describe conduction band of GaN_xAs_{1-x}
(GaNAs). As well known, GaNAs shows exotic behavior such as large band gap
bowing. Although there are various models to describe the conduction band of
GaNAs, origin of the band gap bowing is still under debate. On the basis of
perturbation theory, we show that the behavior of conduction band is mainly
arising from intervalley mixing between Gamma and L or X. By using
renormalization technique and group theoretical treatment, we derive a reduced
Hamiltonian which describes well the band gap shrinkage of GaNAs.
