Shell-core Cu2O-Cu nanocomposite particles and metal Cu nanoparticles are synthesized in a new microemulsion system which consists of saturated Cu2+ salt aqueous solution dispersed in isopropanol and stabilized by polyvinylalcohol (PVA). The size of the composite particles and the thickness of the Cu2O shell layer can be controlled by the volume ratio of isopropanol to H2O (the ratio is defined as R). When R greater than or equal to 1000, it is available to obtain metal Cu nanoparticles.