In situ synthesis of copper-containing nanoparticles in polyethylene matrix by the method of thermal decomposition of (CH3COO)2Cu.H2O was presented. Average dimensions, crystalline structure and size distribution of coppercontaining nanoparticles were determined by transmission electron microscopy (TEM) and X-Ray diffraction. Effects of wt percent of Cu to size of nanoparticles, composition and crystalline structure were investigated in detail. It was shown that when copper concentration of lower 6 wt percent, the nanopartiles are mostly CuO and Cu2O particles. When the sample containing copper concentration of higher 6 wt percent, the nanoparticles contain Cu2O and metallic Cu. With the sample containing 8 wt percent of copper, the obtained Cu2O and Cu nanopartiles are uniform with average sizes of 10 to 30 nm.