Cuprous oxide (Cu₂O) microcrystals with systematic shape evolution were successfully synthesized via a facile microwave-assisted heating technique. The monodispersed Cu₂O was synthesized using copper acetate as a starting material, ethylene diamine tetraacetic acid (EDTA) as a reducing agent and surface-regulating agent and a mixture of water and n-butyl alcohol as reaction solvent. Various morphologies of Cu₂O microcrystals, including octahedral, truncated octahedral, cuboctahedral, truncated cubic and cubic microcrystals, were obtained by altering the volume ratio of n-butyl alcohol to water. The morphologies and optical properties of the synthesized Cu₂O microcrystals were characterized by XRD, SEM, TEM, HRTEM, SADE and UV-Vis/DRS. The growth mechanism of these crystals was thereby proposed. The volume ratio of n-butyl alcohol to water in the reaction medium was a critical factor in precisely controlling the morphologies of the microcrystals. Furthermore, their comparative photocatalytic activities for the degradation of methyl orange were tested.