were grown under high pressure using sodium hydroxide (NaOH) and potassium hydroxide (KOH) with a molar ratio of 1:1 as a flux. The growth mechanism of Cu{sub 2}O polyhedral microcrystals are proposed and discussed. - Graphical Abstract: The Cu{sub 2}O crystals with truncated octahedral shape were one-step synthesized in high yield via high pressure flux method for the first time, which is green and environmentally friendly. The mechanisms of synthesis and crystal growth were discussed in this paper. Highlights: > Cuprous oxide was one-step green synthesized by high pressure flux method. > The approach was based on the reverse dismutation reactions between cupric oxide and copper metal. > This progress is green, environmentally friendly and energy efficient. > The synthesized Cu2O crystals were of truncated octahedra morphology.