Theory and experiments suggest that the oxygen isotope (¹⁸O) ratio of leaf organic matter tends to be lower in highly-transpiring plants than in less-transpiring plants when they grow under the same environmental condition. However, the theory still has some uncertainties when applied to trees in the field conditions. We collected several leaves after measuring their transpiration rate and stomatal conductance in temperate deciduous and evergreen trees (Cinnamomum camphora Linn. and Prunus x yedoensis Matsum.) growing in Mie, Japan once per 4-30 days from June 2009 to October 2010. The area, dry mass and ¹⁸O ratio of the leaf samples were measured. Air temperature, relative humidity and photosynthetic photon flux density were also measured. The transpiration rate and stomatal conductance were mostly higher in P. y. than in C. c. The ¹⁸O ratio of leaf organic matter was always lower in P. y. than in C. c. except during the period that the leaf dry mass per area (LMA) was increasing. These differences between two species agreed with the theory. However, the seasonal variation of the ¹⁸O ratio of leaf organic matter did not reflect that of the stomatal conductance.