Abstract: Nocturnal transpiration, as a consequence of incomplete stomatal closure, is prevalent across species and environments, and high nocturnal atmospheric vapor pressure deficit (VPD) along with high soil water availability are the most commonly reported environmental drivers of it, which is coincided with the conditions of riparian forest in extremely arid region. Based on the measurements of xylem sap flux, leaf gas exchange, stomatal microstructure and environmental factors, we confirmed the occurrence of nocturnal transpiration in P. euphratica from followings: 1) incomplete stomatal closure was observed and mean stomatal conductance was 45 mmol/(m²·s), greater than the minimum stomatal conductance of Populus spp. reported in the literature (approximately 5 mmol/(m²·s)), and along with mean transpiration of 0.7 mmol/(m²·s), both accounting for 26% and 17% of daytime, respectively. This suggested that high stomatal conductance and transpiration are largely resulted from stomatal opening. 2) On average, sap velocity was 31.3 cm/hour at daytime and 16.5 cm/hour at nighttime, which accounting for 53% of daytime. Whether during daytime or nighttime, sap velocity was logarithmic positively related with VPD, and this could explain 55% of nighttime sap velocity change, suggesting that nighttime sap flow was composed of transpiration and tissue refilling. Thus, further research to distinguish the nocturnal transpiration and tissue refilling is needed. Noteworthly, the abruptly increased sap flow after midnight was synchronized with the stem radius change and water potential, but oppositely with VPD, showing that increased sap flow after midnight was induced by tissue refilling but not transpiration.