Heterogeneous nucleation of water vapor on monodisperse submicrometer particles is investigated in a how cloud chamber. The size dependence of critical supersaturation in the range of 15 to 120 nm is experimentally determined. Three types of aerosol, SiC, SiO2, and naphthalene, are tested. The results show that all three aerosols induce heterogeneous nucleation much better than that predicted based on bulk physical properties and, for SiC and SiO2, even better than perfectly wetted particles. The experimental critical supersaturation is smaller than that predicted by the Fletcher version of the Volmer theory of heterogeneous nucleation even with the line tension and surface diffusion taken into account and has a size dependence in qualitative agreement with that theoretically predicted but to a lesser degree. The discrepancy cannot be fully accounted for by the effects of line tension and surface diffusion and the existing theory concerning the curvature dependent physical properties. We conclude that the macroscopic theory of heterogeneous nucleation leads to significant underestimation of the nucleation rate and prediction of higher critical supersaturation than that experimentally measured.