The leaf epidermis of sweet potato, Ipomoea batatus Lam., was peeled to make a direct measurement of photosynthetic rate of mesophyll tissue without stomatal regulation, using the assimilation chamber method. Before peeling the epidermis. transparent tape was affixed on the upper side of the leaf in order to reinforce its physical strength. Then highly adhesive cloth tape was affixed on the lower side of leaf. When the cloth tape was removed from the leaf, the lower epidermis was peeled off with it without damaging the mesophyll tissue. The peeled area was a little larger than 2.5 cm × 2.5 cm. Photosynthetic rates of the leaf, before and after peeling, were measured using a portable assimilation chamber. Photosynthetic rate in peeled leaf was much higher than in unpeeled leaf. The rate of photosynthetic increase, however, was different, depending on the stomatal aperture and photosynthetic rate measured before epidermal peeling. In general, the higher increase rates were found in leaves with smaller stomatal aperture. This suggests that the mesophyll tissue may retain a considerably high photosynthetic potential even if the stomata are closed. There was a rectangular hyperbolic relationship between effect of epidermal peeling (photosynthetic rate of peeled/unpeeled leaf) and stomatal conductance determined before epidermal peeling, and a clear changing point was found in the stomatal limitation to gas exchange ; the peeling was greatly effective at the stomatal conductance below 0.2mol m-2s-1, and almost ineffective above 0.2mol m-2s-1