Different types of ionic liquid-based nanofluids have been studied by the researchers since the last two decades. Most of the studies have focused on the effect of nanoparticles on the enhancement optical properties (absorption, transmittance, scattering, and extinction coefficient) of the corresponding ionic liquid-based nanofluids. However, optical properties of ionic liquid-based fluid have considerable contribution to absorbance in nanofluids. Therefore, it is necessary to study absorption properties of the different ionic liquid-based fluid of corresponding nanofluids to utilize ionic liquid-based nanofluids in solar thermal applications. The effect of the anion and cation radius in the ionic liquid on the enhancement absorption properties of the corresponding nanofluids is investigated. Through the measurement results, we find that as the cation radius or anion radius increases, the transmittances of the corresponding nanofluids decrease sharply, which is beneficial to enhance absorption properties of Fe3O4/ionic liquid nanofluids. At the same time, we also find that the large viscosity increase the stability of ionic liquid nanofluids, which can affect the absorption properties. Understanding the effect of ILs base fluid on enhancement absorption properties of the corresponding nanofluids will pave the road for improving the efficiency for direct absorption solar collectors.