Abstract : We have investigated the electric-field intensity distributions and characteristics of the whispering-gallery mode (WGM) from a planar microdisk using a three-dimensional finite-difference time-domain method. Silicon nitride (Si3N4) planar microdisks on silica (SiO2) substrates have been systematically analyzed considering the effects of the sidewall angle, etching depth, and height. Superpositioning of the TE and TM modes which is caused by the skew effect according to the sidewall angle has been demonstrated. Furthermore, the mode splitting caused by the change from single mode to multi-mode regimes due to an increase in the height has been analyzed. Several planar microdisks of Si3N4 on SiO2 were fabricated, and their resonance characteristics were probed by using micro-photoluminescence spectroscopy. A quality factor of 5×103 for microdisks with a diameter of 3.5 μm and sidewall angle of 35° was observed in the visible range. The WGMs of fabricated microdisk were analyzed according to the details of the model and found to be in good agreement