Abstract: The pine forests were damaged in various degrees because of the drought in Yunnan Province in recent years.Water potential of pine needle is an important index of water content of pine trees,which reflects the water metabolism of pine trees to a certain degree.Pine needles′ water potential is closely related to the pine trees′ growth and adversity resistance and is one of the main parameters for estimating how water stress influences the drought resistance of the trees.The daily fluctuation of water potential in needles from different positions of Yunnan pine (Pinus yunnanensis Franchet) trees was measured during different stages of a dry season in this study.The relationships between needles′ water potential and the changes of illumination,temperature or humidity were also analyzed and discussed.The results indicated that the diurnal variation of needles water potential was affected by biotic and abiotic factors such as trees height,illumination,temperature and humidity.The results also showed that needles water potential was positively correlated to humidity,but negatively correlated to temperature,illumination and the pine needles′ positions on a tree.The water potential had a diurnal decrease and a nocturnal increase,which peaked from 6 a.m to 8 a.m and declined to the minimum at 2 p.m.