We measure heat transfer coefficients of natural convection between two vertical smooth parallel plates heated uniformly in the laminar, transition and turbulent regions. The heat transfer characteristics are experimentally investigated with changing width, δ, between the vertical parallel plates, heat flux, qω, length, L, of the vertical parallel plates and heating conditions variously. For natural convection between the vertical parallel plates, in the turbulent region of Ra*x>1014, the heat transfer is strongly suppressed owing to the effect of combined convection. On the contrary, the heat transfer in the laminar region is enhanced owing to the tunnel effect. These tendencies become marked with decreasing δ and increasing L. The position occurring heat transfer reduction shifts to the downstream with increasing qω under a fixed δ. Furthermore, under smaller δ, we can not clearly distinct the transition process in accordance with both the heat transfer enhancement in the laminar region and the heat transfer reduction in the turbulent region.