For a trap-free single layer single carrier (hole-only or electron-only) organic device of thickness d, dielectric constant ε_r, and mobility μ, the relation of current density j with voltage V is described by the well-known Mott–Gurney equation, [Formula: see text], which can be rewritten as [Formula: see text], where [Formula: see text] can be called space-charge limited (SCL) conductance. We investigated the current–voltage characteristics of vertical organic heterojunction phototransistors based on graphene as the source and C₆₀/copper phthalocyanine (CuPc) heterojunction as the photoactive layer. We found that the drain current vs drain voltage ( I_d- V_d) characteristic is composed of two distinct SCL conductions with SCL-conductance being strongly dependent on the gate voltage and illumination intensity. At low gate voltages, the I_d- V_d curve can be divided into two sections of SCL current conduction with different SCL-conductance: the lower conductance in the lower drain voltage range and the higher conductance in the higher drain voltage range. Both low and high SCL-conductance increase with the gate voltage and illumination intensity. However, as the gate voltage increases to a certain threshold, the two SCL sections unify to only one with the conductance being between them. Our findings implicate that the current conduction of an ideal vertical organic phototransistor (VOPT), whose source/organic interface contact is Ohmic and organic semiconductor is trap free, can be well modeled by the SCL conduction theory with carrier density dependent mobility, which is strongly related to the gate voltage and illumination intensity, and that the mobility of VOPTs at a certain gate voltage can be extracted by the Mott–Gurney equation.