Thermally induced acoustic oscillations in a pipe have been studied analytically and experimentally. Temperature distributions have been obtained near a heat source in a uniform flow with an acoustic field by solving heat-conduction equations. The temperature change of gas causes a volumetric change, and it may convert thermal energy to acoustic energy under certain conditions which have already been clarified only qualitatively. Using the calculated temperature distributions, the amount of converted energy in a cycle has been computed and proved to be a function of the heat source location, the current velocity, the frequency and the thermal diffusivity of the fluid. Experimental results agreed well with the analysis where the modified current velocity is used considering the boundary-layer around the heater. Some other characteristics of the oscillation have been discovered ; for example, the growth rate of oscillation changes when turbulent transition occurs over a certain amplitude.