  We suggest that the phase transition observed in NaV_2O_5 at T_c = 34K is not
a spin-Peierls transition, but a charge ordering transition, related to the
formal presence in this system of equal number of V^4+ and V^4- ions. Below
T_c, V^4+ ions form a zigzag structure, which is consistent with the
experimentally observed doubling of the lattice period in a and b directions.
We show that this charge ordering also results in the alternation of spin
exchange constants along the b-direction, which opens a gap in the spin
excitation spectrum. We emphasize the role of lattice distortions around V ions
both in the formation of the charged ordered state and in the spin-gap opening.
