Abstract: Ultrasonic techniques offer several possible strategies for performing liquid level measurements. One possible solution is to use a pulse-echo measurement of reflection from the liquid-container interface by sigle probe for large container. The improvement of the difference of reflection coefficient between container-liquid and container-air interface is the key technology to enhance the detection sensitivity. Theoretical analysis on the reflection of longitudinal wave incidence obliquely in two types interface was performed based on Snell theorem and the boundary condition when the ultrasonic impedance of container material and liquid medium were fixed in some application.The functional relationship graphics between reflection coefficient and incidence angle in ceramic-silicone oil and ceramic-air interface were given. This is an example used in porcelain-bushing oil level measurement. In addition, there is an optimal incident angle for the incidence longitudinal wave with the maximum difference in the two type interface above mentioned, which is equal to 24.36°. The method would be a useful strategy for performing liquid level measurements, especially to the situation of low impedance liquid inside sealed containment vessels.