We estimated leaf vapor conductance (g l) of cucumber grafted transplants under greenhouse growing conditions. Fifty-six transplants were placed on a bench in the greenhouse. The transpiration rate (Tr) of the canopy was estimated by weighing the 16 transplants in the center using an electronic balance. The total vapor diffusion resistance (R l+b) from inside the leaf to the atmosphere was estimated based on the vapor diffusion model, which incorporates the absolute humidity near the leaf surface and that inside the leaf as well as Tr. Next, g l was estimated from R l+b and the resistance of leaf boundary layer evaluated with a model leaf. The Tr in the afternoon tended to be larger than that in the morning at the same photosynthetic photon flux (PPF) level. By contrast, the g l in the afternoon tended to be smaller than that in the morning at the same PPF level. The decrease of g l in the afternoon seems to be induced by the excessive transpiration resulting from an increase of vapor pressure deficit at the leaf surface.