Abstract: The transp iration variation and relationship between supp ly and demand of water for the Eucomm iaulm oides p lantations in the rocky mountainous area of North China were analyzed based on the experimental data oftree sap flow, canopy, micrometeorological factors and rainfall. The trees were 10 years old. The experiment wasconducted in Ap ril2Sep tember of 2005 and 2006. The aim of this study is to p rovide the essential evidence aboutwater ecology for develop ing E. u lm oide p lantations in the area. The main results are as follows: ( 1) The diurnalvariations of E. ulm oide p lantations transp iration ( TR ) were demonstrated as a single-peak curve in clear day. Incloudy day, itwas demonstrated as a multi-peak curve. In overcast and rain day, it was demonstrated as a multipeak curve and much less than that in clear or cloudy day. The daily variations of transp iration ( TR ) wascharacterized with that TR rosed gradually in Ap ril, and reached a peak in June, fell down readily in July. In 2005 and 2006, the total TRs ( ToTR )were 268. 5mm, 212. 0 mm in the main season (Ap ril2Sep tember) , respectively. InAp r, May, Jun, Jul, Aug and Sep, the ratio of the monthly TR to ToTR were 12. 3% , 20. 4% , 20. 2% , 18. 2% ,15. 9% and 13. 0% in 2005, 10. 6%, 21. 7% , 23. 3% , 17. 4% , 14. 6% and 12. 4% in 2006. (2) In allmonthsduring the main growth season of 2005 and 2006 , TR showed significantly compound correlated with the solarradiation (Ra) , air temperature, humidity, and wind speed over the canopy (α = 0. 01) , and Ra was the mostimportantmicro2meteorological factor effecting on TR through comparing the partial coefficient; (3) In terms of thewhole growth season, the ratio of rainfall to transp iration ( P / TR ) was higher than 1. 0 excep t in Ap ril. So, therewas no water supp ly and demand conflict, or water shortage and water stress as for the main growth season in theexperiment area.