Abstract: Photosynthetic physiology of Dendrobium williamsonii and D. longicornu were studied. Cross-sections of the leaves of 2 Dendrobium species showed that they were bifacial leaves. The abaxial epidermis had large amount of stomata, which covered by the waxy stomatal cover, the vascular bundles had no chloroplast and no ‘Kranz’ structures. Their light compensation points (LCP) were 5-10 μmol?m?2?s?1, the light saturation points (LSP) were ranged from 850 to 900 μmol?m?2?s?1, max photosynthetic rate was around 6 μmol CO2?m?2?s?1; the CO2 compensation points (CCP) and the saturation points (CSP) were 80-90 μmol?mol?1 and 800 μmol?mol?1, respectively, the optimum temperatures of the photosynthesis were in range of 26-30°C. The Pn diurnal variations of 2 Dendrobium species were all bimodal curves, with the first peaks ca 5-6 μ mol CO2 m?2 s?1 and emerging at between 11:00-12:00, and the second peaks emerging at about 15:00, and no CO2 absorption occurred in night. 2 Dendrobium species both had relatively high RuBPCase and GO activities, but very low PEPCase activities. All results suggested that Dendrobium chrysanthum and D. dixanthum were typical semi-shade C3 plants.