Polyoxometallates Co(en) 3PMo 12O 40·8H 2O(+,-)[abbreviated as PMo 12-Co(+,-)], nanoparticles were synthesized by the chemical precipitation reaction at room temperature for the first time. They were characterized by means of elemental analyses, IR spectrometry and X-ray diffraction. The images of scanning electron microscopy(SEM) and the results of calculation by using the Scherrer equation for the line widths of the XRD patterns were used to estimate the average particle size of the powder products, which was 40 nm. The results show that the nanoparticle size was affected by starting materials′ concentrations. The particles had a small size and a narrow distribution, when the concentrations of H 3PMo 12O 40·24H 2O and Co(en) 3I 3·H 2O were around 1.7×10 -4 and 1.0×10 -3 mol/L, respectively. When the concentration was increased, there was no significant increase in the particle size, but more polydisperse Co(en) 3(PMo 12O 40) (+, -) were obtained. In poly(ethylene glycol)(PEG) with an average molecular weight(M W) of 600 g/mol and containing LiClO 4[n(O)/n(Li)=100∶1] as the supporting electrolyte, the conductivity of the composite system increases upon the addition of PMo 12-Co(+ or -) nanoparticles, which was measured by the a.c. impedance technique. The interactions among PEG, LiClO 4, PMo 12, and Co can be used to explain that the PMo 12-Co(+ or -) nanoparticles could promote the conductivity of the PEG-LiClO4-PMo12-Co system. Polyoxometallates Co (en) 3PMo12O40.8H20 (abbreviated as PMo12-Co (+, -)], nanoparticles were synthesized by the chemical precipitation reaction at room temperature for the first time. They were characterized by means of elemental analyzes, IR spectrometry and X-ray diffraction. The images of scanning electron microscopy (SEM) and the results of calculation by using the Scherrer equation for the line widths of the XRD patterns were used to estimate the average particle size of the The results show that the nanoparticle size was affected by the starting materials’ concentrations. The particles had a small size and a narrow distribution, when the concentrations of H 3PMo 12O 40 · 24H 2O and Co (en) 3I 3 · H 2O were around 1.7 × 10 -4 and 1.0 × 10 -3 mol / L, respectively. When the concentration was increased, there was no significant increase in the particle size, but more polydisperse Co (en) 3 (PMo 12O 40) (+, -) were obtained . In poly (ethylene glycol) (PEG) with an average molecular weight (MW) of 600 g / mol and containing LiClO 4 [n (O) / n (Li) = 100: 1] as the supporting electrolyte, the conductivity of The interactions among PEG, LiClO 4, PMo 12, and Co can be used to explain that the PMo 12 -Co (+ or -) nanoparticles could promote the conductivity of the PEG-LiClO4-PMo12-Co system.