Both the cubic single crystals and the spherical polycrystals of cuprous oxides (Cu2O) were prepared by the reduction of aqueous CuSO4, with hydrazine as a reductant in the presence of different organic molecule additives, such as hexadecyltrimethyl ammonium bromide (CTAB), glucose (C6H12O6), ethylene glycol (C2H6O2), and glycerol (C3H8O3), at room temperature. It was found that the organic additives added had an important influence on the formation mechanism of the Cu2O particles and could lead to different microstructures of the resultant Cu2O crystallites. The addition of CTAB to the reaction solution could induce the formation of cubic Cu2O single crystals, while glucose resulted in the formation of spherical polycrystals. All the samples were characterized by X-ray powder diffraction and transmission electron microscope (TEM). TEM observations indicated that the nanosized primary particles formed immediately after the reactants were mixed, followed by the competition between diffusion of the reactive constituents and aggregation of the nanosized primary particles. In the presence of CTAB, the diffusion of the reactants was a major factor contributing to the single crystal morphology of the Cu2O products, while the aggregation of the primary nano-particles into polycrystallites prevailed when glucose existed in the reaction system.