Arrays of InGaN MQW/GaN micro-cone cavities with a base diameter of 3.3 mum were fabricated by ion beam etching. The micro-cones consist of a 58 nm thick multiple quantum wells (MQW) of In0.22Ga0.78N/In0.06Ga0.94N as well as a 1.5 mum thick epilayer of GaN. By using a novel optical ray tracing method, we have figured out four main types of optical resonant cavities inside the three-dimensional micro-cone, including two Fabry-Perot modes types as well as two whispering gallery modes types. Optical resonant modes from a single micro-cone could be clearly observed in the photoluminescence spectra at temperature up to 200 K under a pumping power density two orders of magnitude lower than that for the III-nitride semiconductor micro-disk or micro-ring cavity. The corresponding mode spacings of the experimental results agree well with the calculated ones. The advantages of this new class of cavity are discussed. These findings are expected to have impact on the design of the UV/blue micro-cavity laser diodes. (C) 2002 Elsevier Science B.V. All rights reserved.