To establish an efficient mass propagation method using stem cuttings of virus-free sweet potato [Ipomoea batatas (L.) Lam.] plantlets, the growth characteristics of four cultivars ('Matnami', 'Shincheonmi', 'Yeonhwangmi', and 'Shinzami') influenced by plug cell size and pinching out followed by layering were investigated. Shoot tip cuttings of 5-cm length were grown in plug trays with 50, 72, 105, and 128 cells filled with vermiculite and perlite (1:1, v/v) using the nutrient film technique and the nutrient solution developed by the National Horticultural Experiment Station in Japan for 3 weeks. Shoot tip pinching out (5-cm length) followed by serpentine layerage (SPFSL) was performed after growth for 3 weeks with 10 × 30-cm spacing in nursery soil. The control plantlets were grown with 10 × 10-cm spacing without shoot-tip pinching and layering. Stem growth of plug plantlets was not different among the plug cell sizes up to 2 weeks, but growth after 3 weeks showed the best elongation in 72 cells followed by 50-, 105-, and 128-cell plug trays. Number of leaves, number of roots, and the longest root length were reduced in proportion to cell volume. Fresh weight and dry weight were high in 50-and 72-cell plug trays with more light and soil volume, while they were significantly reduced in 128-cell plug trays. At 6 weeks after transplanting, number of leaves, number of side shoots, fresh weight, and dry weight were significantly increased in SPFSL plants compared to the control. Mean length and number of side shoots (over 5 cm) per plant in SPFSL plants were 1.9 and 1.7 times higher than the control. In this work, the plug cell size for stem cutting was economically suitable in 105-cell plug trays for 2 weeks, and in 72-cell plug trays over 3 weeks. The propagation of stem cutting by SPFSL for farmers was 1.7 times higher than the control.