By stepwise adding of reducer N2H4·H2O,cuprous oxide(Cu2O)nanoparticles(NPs)with adjustable structures were synthesized.The features of Cu2O NPs were characterized by XRD,TEM and UV-Vis absorption spectra.When the reducer was added into the reactant system at one time,the sizes of the Cu2O NPs are in the range of 120–140 nm.Most Cu2O NPs are solid spheres.As the reducer was divided into two equal parts and stepwisely added,almost all the NPs are hollow spheres with good size(150–170 nm)distribution and dispersity.But when the reducer was divided into three or four equal parts and stepwisely added,the NPs are hollow spheres,core-shell structures or solid spheres,and the sizes distribution of the products is deteriorated.The effect of sodium hydrate(NaOH)was also probed.Addition of NaOH speeded up the nucleation and growth processes of Cu2O NPs.With the alkalinity increase,the shells of the hollow spheres become compact and the thicknesses of the shells increase,but the size distribution of the NPs is deteriorated.The absorption spectra of the Cu2O NPs are tunable.With the shell thicknesses increase,the absorption peaks have red shifts.An inside-outside growth model of Cu2O NPs was proposed to explain the results.The Cu2O single crystalline grains grow not only in the reactant solution,but also inside of the hollow nanospheres.The new Cu2O nanocrystallines can not only aggregate onto the shells of the nano hollow spheres,but also inside and outside of the hollow spheres,which leads to increasing the shell thicknesses of the hollow spheres,forming core-shell structures or small solid spheres of Cu2O NPs,respectively. By stepwise adding of reducer N2H4 · H2O, cuprous oxide (Cu2O) nanoparticles (NPs) with adjustable structures were synthesized. These features of Cu2O NPs were characterized by XRD, TEM and UV-Vis absorption spectra. system at one time, the sizes of the Cu2O NPs are in the range of 120-140 nm. Host Cu2O NPs are solid spheres. As the reducer was divided into two equal parts and stepwisely added, almost all the NPs are hollow spheres with good size (150-170 nm) distribution and dispersity.But when the reducer was divided into three or four equal parts and stepwisely added, the NPs are hollow spheres, core-shell structures or solid spheres, and the sizes distribution of the products is deteriorated . The effect of sodium hydrate (NaOH) was also probed. Addition of NaOH speeded up the nucleation and growth processes of Cu2O NPs.With the alkalinity increase, the shells of the hollow spheres become compact and the thicknesses of the shells increase, but the size distrib ution of the NPs is deteriorated. The absorption spectra of the Cu2O NPs are tunable. With the shell thicknesses increase, the absorption peaks have red shifts. Ann inside-outside growth model of Cu2O NPs was proposed to explain the results. Cu2O single crystalline grains grow not only in the reactant solution, but also inside of the hollow nanospheres. the new Cu2O nanocrystallines can not only aggregate onto the shells of the nano hollow spheres, but also inside and outside of the hollow spheres, which leads to increasing the shell thicknesses of the hollow spheres, forming core-shell structures or small solid spheres of Cu2O NPs, respectively.