Modified spontaneous emission properties in the presence of confined photon modes inside the three-dimensional (3-D) optical microcavities are demonstrated, Self-formed pyramidal-shaped semiconductor structures fabricated by selective-area growth technique are utilized as an optical microcavity in which discrete photon modes are generated. Noticeable modification of spontaneous emission from active layers embedded in microcavity structures is clearly observed in mu-PL spectra at room temperature, Almost perfect coincidence between enhanced photoluminescence peak wavelengths and the resonance modes inside microcavity is observed. Furthermore. Purcell factors calculated from the obtained Q values reach similar to9, which is inaccessible in the planar microcavitics with only one-dimensional photon confinement normal to the layers. These results indicate that the effective coupling between electronic system and 3-D confined optical fields is realized by the achievement of present low-loss 3-D microcavities with small cavity volume V-c whose dimension is comparable with lambda(3). (C) 2002 Elsevier Science B.V. All rights reserved.