Healing and acclimatization are key processes for the survival of grafted plants. This study evaluated the influence of light intensity (photosynthetic photon flux, PPF) and relative humidity during the healing and acclimatization period on the photosynthetic characteristics, graft-take, and growth of grafted cucumber (Cucumis sativus L.) seedlings, using a system for the continuous measurement of the $CO_2$ exchange rate, in order to establish optimum environmental conditions for the healing and acclimatization of grafted cucumbers seedlings. Cucumbers (Cucumis sativus L. cv. Joeun Baekdadaki) were grafted onto rootstocks (Cucurbita maxima D. ${ imes}$ C. moshata D. cv. New Shintozwa). Six combinations of two levels of relative humidity (95 and 90%) and three levels (0, 142, and $237{mu}mol{cdot}m^{-2}{cdot}s^{-1}$) of light intensity were set up during healing and acclimatization. Increasing light intensity significantly increased $CO_2$ exchange rates during healing and acclimatization. At 95 and 90% relative humidity, the $CO_2$ exchange rates at $237{mu}mol{cdot}m^{-2}{cdot}s^{-1}$ light intensity were 1.5 and 1.8 times higher than those at $142{mu}mol{cdot}m^{-2}{cdot}s^{-1}$ light intensity, respectively. The light intensity during healing and acclimatization also affected the amount and distribution of chloroplasts in scion cotyledon. The amount of chloroplasts increased with the increase of PPF during healing and acclimatization, which covered most of cell wall with little open space left, compared with that of dark condition. As PPF increased, the shoot length, ratio of shoot to root, and specific leaf area decreased but the hypocotyl diameter, leaf area, dry weight, and percent dry matter increased. On the other hand, the relative humidity ranging from 90 to 95% did not significantly affect the $CO_2$ exchange rates during healing, acclimatization, and growth of grafted cucumber seedlings. As a result, PPF during healing and acclimatization affected the growth and quality of grafted cucumber seedlings. This showed that higher PPF condition may improve the growth and quality of grafted cucumber seedlings.