A simple and potentially scalable sub- and super-critical methanol technique was applied to synthesize cuprous oxide. Cu2O microcrystals with two morphologies were obtained in methanol of subcritical domain by adjusting the feeding methods. The effects of the reaction time, temperature, pressure, and feeding method on the formation of Cu2O microcrystals were investigated. XRD, SEM, and XPS were applied to analyze the composition and morphology of products. Results showed that pure cuprous oxide could be obtained in subcritical methanol of 230 A degrees C, 7.8 MPa for 15 min. The two as-obtained products displayed octahedral and cubic morphology with an average edge length of 4 and 2 mu m by solution and solid feeding methods, respectively. The formation and crystal growth mechanism of Cu2O in subcritical methanol were also proposed on the basis of above results. Furthermore, the two microcrystals were employed as catalysts in the photodecomposition of negatively charged molecule methyl orange. Results showed that both microcrystals possessed excellent photocatalytic activities. However, the obvious difference between cubic and octahedral in photocatalytic performance was not found in the scale of micrometer (2-4 mu m).