We calculate the energy level spacing of InAs/GaAs quantum dots. From the calculations, as the size of quantum clot becomes larger, the spacing between ground state and 1st excited state increases. At a fixed quantum dot diameter, there is a height that gives the maximum level spacing. This interesting behavior comes from the small height to diameter ratio of typical quantum dots. From these results, the InAs/GaAs quantum dot emitting at a longer wavelength is typically the better QD system in view of dimensionality.