In combustion chambers thermo-acoustic instabilities arise due to the coupling between the unsteady heat release and pressure fluctuations. The instabilities created cause serious structural vibration, flame blow out, increase in heat transfer and damage, thereby reducing the life span and performance of jet engines, gas turbines and industrial burners. Therefore, there is need to develop a simple technique for effective control of these instabilities. The present work involves experimental study of thermo-acoustic instabilities occurring in a Rijke tube and their suppression by using Helmholtz resonators of different shape (a passive method of suppression). The combustor model is replaced by Rijke tube which is an experimental setup extensively employed for research purpose. Rijke tube is provided with a co-axial pre-mixed gas burner as the source of heat which could be placed at any desired position. When a heat source with proper air to fuel ratio is placed at about one fourth of the tube length, 3rd mode of instability is obtained and a loud noise of frequency 681 Hz is produced. A comparative study of thermo-acoustic instability suppression using cylindrical, spherical and conical Helmholtz resonators has been performed. Spherical resonator has been found to be the most effective one imparting a maximum damping effect of 20 dB and an average damping effect of 10.5 dB.