We demonstrate the suppressed bimodal size distribution of InAs quantum dots (QDs) grown with an As2 source using molecular beam epitaxy. QDs grown with an As2 source have single mode photoluminescence (PL) spectra, whereas QDs grown with an As4 source have bimodal spectra. The PL intensities of QDs grown with an As2 source are higher than those grown with an As4 source. The density of QDs grown with an As2 source decreases at high As pressure, whereas the density of As4 grown QDs increases with As pressure. These results indicate that the surface migration of In atoms is enhanced at a higher As2 pressure, resulting in improved optical properties. The enhanced surface migration is thought to be due to the stable As–As dimer structures under an As2 source.