Change in the structure of deciduous dipterocarp forest was studied along an altitudinal gradient at Queen Sirikit Botanic Garden, Chiang Mai province. Each of permanent plot of 100 x 100 m2 (1 ha) was established at 700, 800, 900 and 1000 m above mean sea level (MSL). Trees with DBH more than 4.5 cm were identified. There were 46, 52, 61 and 63 species inthe plot of 700, 800, 900 and 1000 m MSL, respectively. The species diversity indices were 3.06, 2.74, 3.07 and 2.95 respectively. Plant community at 700-1000 m MSL was deciduous dipterocarp forest. The dominant tree species in the plot of 700 m MSL were Phluang (Dipterocarpus tuberculatus), Teng (Shorea obtusa), Tumkakhao (Strychonos nux-blanda) and Hiang (D.obtusifolius). Those of 800 m MSL were Phluang, Teng, Ko phae kon (Quercus kerrii) and Mueat lot (Aporosa villosa). In the 900 m MSL plot, it was dominated by Teng, Phluang, Hiang and Ko phae kon. For the 1,000 m MSL plot, the dominant tree specise were Phluang, Ko phae kon, Teng and Rang (S.siamensis). There was also a mixture of Fagaceae trees in this plot. Plant communities at the 800 and 900 m MSL were the most similar, similarity index was 71.19%. In contrast, plant communities of the plots at 700 and 1000 m MSL were obviously different, similarity index was 53.10%. All species in the plot from 700-1000 m MSL were contagious distribution. Altitudinal gradient had negative relationships with exchangeable Ca at soil depth of 15, 30 and 50 cm, total N at 15 cm, organic matter at 15 cm, available P at 15 and 50 cm, pH at 15 and 30 cm and silt at 15 and 30 cm but had positive relationships with clay at 15 and 30 cm, exchangeable K at 15 cm and exchangeable Na at 15 and 30 cm. Altitudinal gradient above MSL were positively correlated with the number of plant species (p<0.05) and percentage of basal area per plot (p<0.01).
