Self-assembled monolayer (SAM) of oxovanadium phthalocyanine (VOPc), an organic semiconductor compound, was prepared at room temperature on a highly oriented pyrolytic graphite (HOPG) substrate by immersing the substrate in a 1,2-dichloroethane (C2H4Cl2) colloidal solution containing both VOPc nanoparticles (2-20 nm) and dissolved VOPc molecules (ca 10-3 g·L^-1). The VOPc nanoparticles further assembled on the VOPc SAM. The nanoparticle-assembling in the experimental conditions exhibited a high size-selectivity to VOPc nanoparticles of (4.60±0.47) nm with a standard deviation of 10%. As revealed by scanning tunneling microscopy (STM) measurements, with increasing VOPc concentration of the colloidal solution from 2.5 ×10^-2 to 2.5 ×10 ^-1 g·L^-1, the amount of VOPc nanoparticles assembled on the molecular layer increased gradually, and a dense VOPc nanoparticle monolayer formed in the end. The found assembly structures and method are promising for the development of photoelectric or electric functional systems.