Effect of light quality on plant temperatures at joining parts of grafted watermelon seedlings was investigated using a thermal imaging system in this study. Plant temperatures at joining parts lowered than those at stem region during graft-taking. However, difference in plant temperatures at joining parts and at stem region decreased by days after graft-taking. Plant temperatures of grafted seedlings graft-taken under white, blue, red, and blue + red fluorescent lamps repeatedly fluctuated high at photoperiod and low at dark period. Considering the variation of plant temperatures at joining pa퍼5, the illumination of blue and red light as well as white light will be effective for grafting of grafted watermelon seedlings. It is recommended that air temperature around grafted seedlings should be maintained at 27 to $29^{circ}C$ for enhancing the physiological reaction of callus and for the smooth joining of scion and rootstock.