Abstract: The role of vessel tissues in the hydraulics and water relationships of woody plants has received increasing attention; however, the traditional devices and methods used for determining their length are relatively cumbersome. We designed a simple device for vessel length assessment based on the principle of gas injection, and simplified the measurement process. Results showed that the designed device successfully assessed the vessel length of Robinia pseudoacacia L. with a dissipation coefficient of 0.111±0.012, maximum vessel length of 56.9±1.5 cm, mean vessel length of 19.6±2.5 cm and mode vessel length of 9.8±1.3 cm. There were no significant differences between the gas injected vessel lengths from either side of the stem. The gas pressure in the injection side did not vary significantly during measurement, and had no influence on the test. The present method was found to be more precise than that of traditional air injection.