Due to environmental consideration, natural ester was developed as liquid insulation for high voltage transformer. This paper reports the Dissolved gas Analysis (DGA) and Fourier Transform Infrareed (FTIR) Spectroscopy of Thermally-Aged Natural Ester-paper composite. The sample used were natural ester from palm oil and kraft paper insulation. The samples used in these experiments were natural ester from palm oil and kraft paper transformer insulation. The 200 gr of ester samples, 10 gr of kraft insulation paper, and 17.5 gr copper were put together with copper conductor in heat-resistant hermetic bottles to simulate actual condition of transformator. The samples were subjected to a thermal stress in a controllable oven at temperature of 120oC and 150oC. The thermal aging was conducted for duration of 2 weeks, 4 weeks and 6 weeks. The dissolved gas in the ester after a certain aging temperature and period was determined using a DGA testing. The gas species and the concentration of each gas under various aging conditions were analyzed. Dissolved gas analysis indicated that thermal aging in ester-paper composite insulation released, CH4 (Methane), C2H6 (ethane), and CO (Carbon Monoxide). FTIR measurement was conducted for the thermally-aged ester insulation. The comparative analysis of DGA and FTIR results was done to clarify the origin of the gas released in the thermally-aged ester paper composite insulation.