Water-based and kerosene-based magnetic nanofluids are prepared, and used to study the photo-thermal conversion efficiency of a solar collector under solar simulator. Iron oxide coated with silica (Fe3O4@SiO2) nanoparticles (NPs) were synthesized and dispersed in two types of base fluids (i.e., water and kerosene) as the absorbing medium. The temperature rise characteristics of the nanofluids are investigated at different heights of the solar collector, for a duration of 300 minutes, under solar intensities of 1000 W/m2 and 2000 W/m2. The experimental results show that Fe3O4@SiO2 nanofluids improve the photo-thermal conversion efficiency compared with the base fluids, since the NPs enhance the light absorption effectiveness. The collector filled with Fe3O4@SiO2/kerosene nanofluid has more temperature intensification than the collector with Fe3O4@SiO2/H2O nanofluid. However, the last one has a higher photo-thermal conversion efficiency, for the reason that H2O has much higher specific heat capacity than kerosene. The overall performance characterization of the nanofluids demonstrated the utility of using nanofluids based on Fe3O4@SiO2 NPs in direct absorption solar collectors.