Dissolved Gas Analysis (DGA) is one of the most effective methods in condition monitoring and fault diagnosis of oil immersed transformers. To facilitate the DGA data interpretation, investigation of fault gas generation under laboratory simulated faults using immersed heating method is often conducted. However, with immersed heating method, it is difficult to generate a stable thermal fault with temperature above 400°C in mineral oil. Therefore this paper introduces an alternative heating method that is able to generate a stable thermal fault with temperature up to 750°C. The tube heating method based DGA thermal fault test system is described in detail and fault gas generations from a mineral oil at various temperatures of 350°C, 550°C and 750°C are investigated using this system.