This study was conducted to examine the low temperature effects on photosynthesis of two Phalaenopsis hybrids 'Hwasu 3551'(HS) and 'White-Red Lip'(WR) at different plant age. Micropropagated clones were acclimated for 4 weeks. After acclimation, plants were grown in a growth chamber at 28/26 o C(HT). Plants were transferred to 21/19 o C(LT) after 0, 2, and 4 months of cultivation at HT, and plant age was 1, 3, and 5 month-old, respectively. The photoperiod was provided by 12 hours with PPF at 110±10μmol•m-2 •s-1 (06:00-18:00 HR). Vegetative growth and diurnal CO2 uptake rate of plants maintained at HT or transferred to LT were measured and compared each other. With all conditions, two Phalaenopsis hybrids showed CAM pathway, which was divided into Phase I to IV by CO2 exchange. When plants were transferred to LT, daily total net CO2 uptake and growth rate decreased. In 3 month-old plants transferred to LT, CO2 uptake rate during phase III decreased to-1.36 and-0.60μmol•m-2 •s-1 in HS and WR(CO2 leakage), respectively. CO2 leakage during Phase III induced significant decrease of daytime total net CO2 uptake, and vegetative growth rate decreased. In addition, when 3 month-old plants were transferred to LT, the number of new leaves significantly decreased. However, when 5 month-old plants were transferred to LT, decrease of number of new leaves were not observed and only leaf length decreased. These results indicated that low temperature effect on vegetative growth and photosynthesis were small in 5 month-old plant, and vegetative growth was maintained. This approach could be helpful for establishing schedule or improving strategy in Phalaenopsis cultivation with energy cost saving.