We fabricated organic p-i-n heterojunction photovoltaic cells of a zinc phthalocyanine (ZnPc)/1:1 codeposition (ZnPc:C60)/C60 structure. We investigated the effects of the intrinsic (i-) layer thickness on the photovoltaic properties. The thickness was changed from 0 nm (=p-n heterojunction) to 50 nm (=all i-layer) with the total thickness of 50 nm. While the short-circuit photocurrent increased with increasing the thickness, the fill factor showed the opposite tendency. Therefore, the power conversion efficiency showed a maximum (1.5%) at the thickness of 10 nm under air mass 1.5 global solar conditions. Device simulation based on idealized equivalent circuit of a solar cell demonstrates that the i-layer thickness is concerned in the series resistance of the cells.