In this study, phenolic compounds were identified in the leaves of grafted and non-grafted watermelon (Citrullus lanatus (Thunb.) Matsum and Nakai) plants and squash rootstocks. The watermelon cultivar 'Crispy' was grafted onto hybrids of C. maxima x C. Moschata, 'TZ-148' and 'RS-841', and Lagenaria siceraria cv. '64 - 18'. The experiments were conducted in a randomized complete blocks design in two time periods. Non-grafted plants were used as the control. Some important differences were detected for the contents of the phenolic compounds during the growing periods such as kaempferol accumulates in the leaves of grafted and non-grafted watermelon plants and rootstocks. Kaempferol contents of the rootstocks were found to be higher than the grafted and non-grafted watermelon plants. Plants grafted on 'TZ-148' or 'RS-841' had higher phenolic content than the others.