An anti-reflection coating (ARC) with sub-micrometer size of structure has brought significant advantage in solar cell efficiency. Owing to the diffraction and light trapping effect caused by a sub-wavelength size of structure, the reduced reflectance covering a wide-wavelength can be expected at a surface of the photovoltaic (PV) cell. We proposed and tried a nano-scale structure fabrication by small-size particles combined with conventional 2 layers ARC on GaAs PV cells. The small-size spherical particles show well aligned self-assembly on the substrate when the solution containing micro-spheres were dried with a proper sheer force. Using the pure water solution of colloidal SiO2 spheres and an ordinal spin-coating technique, a well aligned mono-particle layer has been fabricated on the 2 layer ARC (TiO2 and SiO 2) on a GaAs PV cell. In this research, we tried 3 samples with different size of particles (400 nm, 200 nm and 100 nm in diameter). As the evaluation of PV cell, I-V characteristics under the AM1.5G illumination and external quantum efficiency (EQE) spectra were measured. For each sample, the I-V and EQE were obtained from the same sample: before and after application of particle coating. Measured relative efficiency enhancement of the additional mono-layer coating was +7.1, +5.7 and -0.3 in % for particle sizes of 100, 200 and 400 nm, respectively. As well as enhancement in light incorporation to the inside of a PV cell, there seem to be scattering loss by the particles on the cell. In the range of tested particle size, the smaller particle was preferable to avoid scattering loss and provides a larger efficiency enhancement to GaAs PV cells.