This study was conducted to determine the effects of CO2 concentration on the growth and physiology of rain tree (Albizia saman (Jacq.) Merr), by increasing the CO2 concentration in a greenhouse automated system. The objective of this study was to evaluate the response of rain tree to CO2 in terms of growth and physiology. CO2 at an average concentration of 800 μmol mol-1 was injected daily for 2 h from 9 am to 11 am. The seedlings were placed in a greenhouse during the control trial with a mean ambient CO2 concentration of 400 μmol mol-1. In this study, the entire randomised block design has been applied, and growth was observed every 30 days for 120 days. Almost all seedling growth parameters were significant under elevated and ambient concentrations. The leaf area in the control samples (400 ppm of CO2) was 243.37 cm2, and this value increased to 277.30 cm2 in the sample treated with 800 ppm of CO2. The biomass increased, and the original wet weight ratio and root dry weight of the canopy and the principal (9.06 and 10.12 g, respectively) increased to 9.7 and 16.06 g, respectively, after treatment. Physiology was analysed in terms of relative levels of photosynthesis, stomatal conductance and water use efficiency (WUE). Such parameters increased in the principal treatment of CO2 (800 ppm), whilst the CO2 content and transpiration levels declined. As the CO2 concentration increased, the value of the levels of photosynthesis and stomatal conductance in both samples increased. As the photosynthesis levels increased, the WUE activity increased. However, as photosynthesis levels decreased, the WUE activity also decreased. Transpiration levels but also rely on a certain age if the increased photosynthesis WUE has decreased.