Leaf area of Phalaenopsis white hybrid was estimated by leaf max. length×max. width × ”a factor” dervied from regression coefficient and varied with different leaf size. The factors determined from population of ＜10 cm, 10-20 cm, 20-30 cm and ＞30 cm leaf length ranges were 0.742, 0.781, 0.823, and 0.848 respectively From analysis of variance for testing significance of regression slopes there exists a common slope and that is the pooled factor 0.825. The standard deviation of factor, regerssion equation and correlation coefficient between estimated and actual leaf area all demonstrated the adequency of the model The leaf length and width were used to calculate total leaf area per plant for Phalaenopsis white hybrid grown in phytotron at various day/night temperature After 9-month experiment, plants grown at 30/25℃ increased 1.4 and 3.8 times total leaf area compared with plants grown at 25/20℃ and 20/15℃, respectively. High temperature seemed to promote leaf initiation, growth and senescence and also enhanced leaf elongation that made leaf become slender in shape, thereafter.