The importance of thermal conductivity as a distinguished heat transfer parameter and possibility of enhancement by using the nanofluid systems, led to attention of many researchers to investigate the nanofluid thermal conductivity. In this study, the thermal conductivity of various nanofluids in the temperature range of 15-45 °C with dionized water, ethylene glycol, and mixed fluid with equal proportion of the two fluids was measured as the base fluids by KD2 instrument. The effects of temperature, base fluid, concentration and cluster size for nanoparticles of alumina, silica, copper oxide and silicon carbid on nanofluid thermal conductivity was investigated. In most cases, the thermal conductivity of nanofluids increased linearly with temperature and nanoparticles concentration. Nanoparticles concentration and temperature have the greatest effect on the thermal conductivity of silicon carbid and copper oxid nanofluids respectively. Increasing 40% of the relative thermal conductivity of 0.01 volume fraction of copper oxid-ethylene glyclol nanofluid, was indicated with the 30 °C temperature rise. In this study in order to make nanofluids stable and verifying their quality ultrasonic instrument and zeta potential test have been used, respectively. A minimum point for nanofluid thermal conductivity by increasing the time of vibration and reducing the cluster size of nanoparticles was observed. It seems that according to particles size, there is a competition between two mechanisms affecting the nanofluid thermal conductivity. Nanoparticles aggregation and Brownian motion of the particles acts as the predominant mechanism for large clusters and small clusters, respectively. In addition, a new model based on fractal theory, Wang and Prasher's model, considering the Brownian motion, effect of nanolayer and aggregation, was developed. This present model has the ability of predicting the nanofluid thermal conductivity influenced by temperature, concentration and thermal conductivity of nanoparticles, base fluid thermal conductivity, Brownian motion, particle, cluster and nanolayer sizes. Keywords : Thermal conductivity, Nanofluid, Brownian motion, Aggregation of nanoparticles.