Temperature distribution of the circular heat source was studied by analyzing the heat transfer of the environment of the circular source for OLED. Circular nozzle source was used to fabricate thin organic layer as the organic material in it was heated, vaporized and deposited to the large size panel. A modified heater structure of circular source has been suggested. The results of numerical analysis shows that the modified heater structure can use 15% more powder in a batch than the original heater structure does. Moreover, the modified heater structure can improve the uniformity of organic vapor deposition by controlling the temperature.