We investigated the optical properties of colloidal-synthesized ZnO spherical nanoparticles prepared from 1-octadecene (OD), a mixture of trioctylamine (TOA) and OD (1:10), and a mixture of trioctylphosphine oxide (TOPO) and OD (1:12). It is found that the green photoluminescence (PL) of samples from the mixture of TOA/OD and TOPO/OD is largely suppressed compared with that from pure OD. Moreover, it is found that all spherical nanoparticles have positive zeta potential, and spherical nanoparticles from TOA/OD and TOPO/OD have a smaller zeta potential than those from OD. A plausible explanation is that oxygen vacancies, presumably located near the surface, contribute to the green PL, and the introduction of TOA and TOPO will reduce the density of oxygen vacancies near the surfaces. Assuming that the green emission arises due to radiative recombination between deep levels formed by oxygen vacancies and free holes, we estimate the size of optically active spherical nanoparticles from the spectral energy of the green luminescence. The results are in good agreement with results from TEM. Since this method is independent of the degree of confinement, it has a great advantage in providing a simple and practical way to estimate the size of spherical nanoparticles of any size. We would like to point out that this method is only applicable for samples with a small size distribution.