During photosynthesis, many resistances in the photosynthetic pathway from the atmosphere to chloroplast stroma limit CO₂ diffusion. Some of aquaporins suggested that having CO2 permeate ability, thereby reduce the resistance on the photosynthetic pathway and thus contribute to raising photosynthetic capacity. However, aquaporin’s functions on photosynthesis and their behavior on drought are not well studied. In this study, we focus on ice plant (Mesembryanthemum crystallinum L.) aquaporin, McMIPB, and investigate the photosynthetic capacity and leaf anatomy of the McMIPB overexpressing tobacco in (1) temporal higher VPD condition, and (2) soil water deficit growth condition. McMIPB over-expressing plants showed relatively higher intrinsic water use efficiency in higher VPD condition (~16%), and kept high photosynthetic rate on soil water deficit condition (~113%). Transgenic plants showed almost the same mesophyll constructions as the control plants. Transgenic plants drastically decrease the stomatal conductance on higher VPD. When they were exposed lower VPD in soil water deficit condition, stomatal conductance increased. McMIPB is likely to behave as CO₂ transporter, as well as humidity sensor and may relate the regulation of stomata.