Dendrobium candidum, a peculiar and valuable Chinese herb and Crassulacean Acid Metabolism (CAM) plant, was cultured for 45 days on a sugar-free modified Murashige and Skoog (MS) medium in Magenta-type vessels with the number of air exchanges of the vessel (NAE) being 3.2 h^-1, at a photosynthetic photon flux (PPF) of 80 mmol m^-2 s^-1. A factorial experiment was designed with two levels of CO₂ concentration, 400 mmol mol^-1 (PA treatment) and 1000 mmol mol^-1 (PAC treatment) in the culture room during the photoperiod. The plantlet growth, photosynthesis and state of stomata were compared to the plantlets cultured on sugar-containing MS medium with PPF of 50 mmol m^-2 s^-1 and NAE of 0.2 h^-1, i.e. the PM treatment. Leaf area, fresh and dry weight, and stem diameter were significantly greater in both PA and PAC treatments than those in the PM treatment, although there were no significant differences in the all growth values between both photoautotrophic treatments. Moreover, the photoautotrophic plantlets showed higher net photosynthetic rate, greater chlorophyll content, normal stomata and less microbial contamination, compared to those of the photomixotrophic plantlets. The PM plantlets had widely opened stomata with circular shape in the photo- and dark periods. The daily CO₂ exchange rate (CER) was negative in the PM treatment, and positive in the PA and PAC treatments, but there was no significant difference in the daily CER between the PA and PAC treatments. The ratio of daily CER in the photoperiod was 16% in the PA without CO₂ enrichment, and 27% in the PAC with CO₂ enrichment. Thus, D. candidum should be considered as a facultative CAM plant. The photosynthetic pathway could change between CAM and C₃ with variation of environmental factors.