A new approach, that combines the photolithography and conformal deposition techniques, was proposed to fabricate Si-based three-dimensional optical microcavities on patterned substrates. Different from the lateral optical confinement of 3D microcavities by using total internal reflection, Bragg reflectors are used for all three-dimensional optical confinement. From the room temperature photoluminescence spectra, discrete optical modes with obvious side-dependence were observed. With the lateral size decreased from 4.5 mu m to 1.5 mu m, the modes shift to higher energies and the mode splitting increases, which indicates that 3D optical microcavities act like photonic quantum dots. The numerical calculations of quantized photon states in photonic quantum dots show a quantitative agreement with these observed discrete optical eigenmodes. (C) 2007 Elsevier B.V. All rights reserved.