In this work we propose a simple method for quality analysis of transformer oil in hermitically sealed aluminium tank using ultrasonic lamb wave sensor. Lamb waves generated using acrylic wedge coupled with ultrasonic transducer attached to the tank get reflected from the top edge of tank and are received at ultrasonic transducer. Lamb wave is attenuated with respect to the mechanical properties of the liquid inside the tank. Using Fast Fourier Transform analysis the magnitude of received A0 mode signal is obtained. Acoustic impedance of the liquid inside the tank is determined using the reflection coefficient calculated from FFT magnitude of A0 mode signal.Average acoustic impedance measured experimentally for one year used oil is 4.0021 MRayls and new oil is 2.882 MRayls. Experimental results are compared with the acoustic impedance calculated with density and viscosity determined using other methods and results are in good agreement. The experiment is repeated for 10 times and found consistent result with a standard deviation of 0.056 for one year used oil and 0.112 for new oil. This technique can be developed as new and efficient method to monitor the quality of transformer oil in hermitically sealed tanks in remote locations.