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The paper presents a method to detect the degradation level of transformer insulating oil. Degradation of transformer insulating oil is due to both mechanical and chemical deterioration. Metallic particles and organic material, such as a dust, are known as the main reasons for mechanical deterioration. According to the degradation and oxidation of insulating oil over the time of transformer operation, the total acid number (TAN) increases, sludge appears, and finally conductivity decreases. An inter-digit capacitive type sensor was designed, and fabricated using MEMS technology. The size of the sensor is 9 mm (length) × 7 mm (width) × 1 mm (thickness). From test results, when the oil is aged, the capacitance increases and the impedance decreases. The result shows good proportionality with time.