Copper(II) oxide ultrafine particles are of great interest as a new material for multiple applications. This paper expounds the synthesis and characterization of copper(II) oxide submicrometer particles formed by a facile and simple supercritical hydrothermal synthesis method. The copper(II) nitrate, copper salt, was used as a precursor aqueous solution heated by a preheated sand bath to reach supercritical conditions. The effects of process operating parameters, such as temperature, pressure, the addition of sodium hydroxide, and precursor concentrations on the morphology and the size of copper(II) oxide submicrometer particles have been investigated. The copper(II) oxide particles formed, with particle sizes of ca. 100 nm, were hexagon-flake-like and spindle-like and free of impurities. The average particle size decreased with the increase of temperature under subcritical conditions and decrease of pressure. It decreased with the increase of precursor concentration at lower concentration conditions and revealed an adverse trend at higher concentrations. Sodium hydroxide accelerates the conversion of copper ions.