MWCNT nanofluid have superior thermal conductivity and optical absorption characteristics than other material. In this study, optical properties of MWCNT nanofluid were investigated according to the concentration of nanofluid in order to improve the performance of the direct solar collector. In experimental results, the transmittance of MWCNT nanofluid was decreased in wavelength range of ultraviolet and visible light according to increasing the MWCNT concentration. Particularly, the transmittance was remarkably improved in range of visible ray. It was over 85 times more with 300 to 400 nm. Therefore, MWCNT/EG nanofluid is suitable to use the working fluid of the direct solar collector.