Abstract: Three kinds of plug trays were used to grow watermelon seedlings which were grafted on shoot with root cutting. The results showed that there was no significant difference among treatments when the seedling age was 20 days after being grafted. Plug cell sizes at 78.4 cm³, 82.9 cm³ restrained seedling growth and root vigor when the seedling age was 25 days after being grafted. Seedling root length, root dry weight, leaf area index, and root vigor were noticeably correlated with plug cell size. It is suggested that the best seedling age is 20 days and no longer than 25 days for the plug cell size of 78.4 cm³, 82.9 cm³. Seedling age can be 30 days for the plug cell size of 131.6 cm³.