Bimodal nanocrystalline (nc) Cu-Ag and nc Cu were prepared by high pressure sintering method in argon atmosphere. The microscopic structures of the samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The thermal conductivity of the samples with average grain sizes ranging from 50 to 270nm were measured. The test results showed that the thermal conductivity of nc Cu-Ag and nc Cu were reduced significantly from 243.84 W/m.K to 163.45 W/m.K and 259.93 W/m.K to 180.36 W/m.K at 300K, respectively. In addition, the thermal conductivity increases with the increasing of grain size. For a better understanding of the effects of grain boundary and size on the thermal conductivity of nc material, a modified model, with special emphasis on the contributions of electron conduction, is presented by incorporating the concept of the Kapitza resistance into an effective medium approach. The theoretical calculations are in good agreement with our experimental results.