This work is an experimental study using a gas ultrasound sensor to investigate the effect of the ultrasonic-assisted measurement on sensitivity and attenuation in pure methane (CH4) gas. This paper is mainly focused on the ultrasonic transmission and reception voltage characteristics, self-vibration, envelope signal, and amplitude. The ultrasonic-assisted measurement system is used for a reorganization purpose of constant volume chamber (CVC) located in an NG or CNG vehicle. The CVC system is designed with two different ultrasonic sensors: pressure regulator, pressure sensor, temperature sensor, safety valve, mass flow controller and actuator, LabVIEW cDAQ and software, and air cylinder for sensor distance control. The experiments are performed in a wide range of operating conditions, i.e., initial pressure of 1–5 bar, temperature of 293 K, initial voltage of 300–500 V, sensor distance of 30–90 cm, resonant frequency of 56.2 kHz, and pulse rate of 7 Hz. As a result, as pressure increases, the measured width of the transmission sensor (T: EVA) and receiving sensor (R: EVA) is remarkably widened in a main measuring signal including envelope amplitude. Also, in the case of a high sensitivity ultrasonic sensor (HSUS) capable of measuring CH4 gas inside a CVC, the measuring sensitivity has a higher efficiency in high-density conditions as well as low-density conditions compared with other types. Consequently, as the initial pressure increases, the entire ultrasonic sensors can obtain higher efficiency and sensitivity in CH4 gas space because of the significantly accumulated gas particles.