Abstract: Nanofluid is an innovative heat transfer fluid with superior potential for enhancing the heat transfer performance of fluids. Recent developments in nanotechnology indicate that the nanofluid is an efficient working fluid and coolant in the solar thermal application. In this paper, heat transfer characteristics of several kinds of the nanofluids as the cooling medium were studied via Mixture multiphase models in Fresnel high concentration system based on the helix microchannel cooling structure, which was used to simulate the reduction of the temperature of GaAs cell with the nanofluid as cooling medium. And the enhancement heat transfer factor was introduced to determine the heat transfer efficiency of the cooling medium. The experimental results showed that the temperature of the microchannel inlet cross-section with the nanofluid as the cooling medium was lower than the distilled water as the cooling medium. Meanwhile, other various factors also affected the microchannel heat transfer characteristics including the mass fraction, particle size and the particle type of the nanofluid. The results displayed that the heat transfer characteristics of Al2O3-H₂O nanofluid increased with the increase of the mass fraction. When the mass fraction was 5.5%, Nusselt number of the nanofluid reached the maximum value. But when the mass fraction was more than 5.5%, the Nusselt number decreased and tended to remain in a stable value around 3.23. Nusselt number of the Al2O3-H₂O nanofluid decreased with the particle size increasing with the same mass fraction. The heat transfer capability of the nanofluid decreased when the mass fraction of Al2O3-H₂O nanofluid was larger than 5.5%, which was mainly attributed to the resistance effect of the viscosity over the high heat conducting ability of the nanofluid. Nusselt number of all the fluids which were the distilled water, the different mass fractions of Al2O3-H₂O nanofluid and SiO2-H₂O nanofluid increased with the increases of their Reynolds number, and Nusselt number of the nanofluid was greater than the distilled water's. Nusselt number of nanofluid increased along with the increases of the mass fraction with the same Reynolds number, and the Nusselt number of Al2O3-H₂O was greater than of SiO2-H₂O, which meant that the heat transfer characteristics of the Al2O3-H₂O nanofluid was better than that of SiO2-H₂O nanofluid. The enhancement heat transfer factor of the nanofluids increased with the growing of the inlet velocity. When the inlet velocity was 0.82 m/s, the enhancement heat transfer factor of Al2O3-H₂O (1%) nanofluid and SiO2-H₂O nanofluid both reached the maximum. But the enhancement heat transfer factor of Al2O3-H₂O nanofluid with the mass fraction was 5% which was proportional to the inlet velocity, and the optimum enhancement heat transfer factor of Al2O3-H₂O(5%) nanofluid was obtained compared with all kinds of the nanofluids when the velocity of nanofluid was lower than 0.68 m/s. Furthermore, the enhancement heat transfer factor of the same kind of nanofluid increased with the mass fraction increasing as the same inlet velocity, and the enhancement heat transfer factor of Al2O3-H₂O nanofluid was higher than that of SiO2-H₂O nanofluid. The heat transfer characteristic of Al2O3-H₂O nanofluid decreased with the particle size growing. This work provided a theoretical reference for the nanofluids applied in the GaAs solar cell cooling under high concentration.