Multiplication ratio (number of average of harvestable propagules per stock plant, M) and electric energy consumption per propagule (E_p) at four levels of planting density (59, 118, 236 and 473 m^-2) were investigated for sweetpotato (Ipomoea batatas (L.) Lam.) vegetative propagation in a closed transplant production system. After n multiplication cycles, total number of harvestable propagules (N_n) and electric energy consumption per propagule (E_{p, n}) in the system were estimated by using mathematical models. Single nodal cuttings were used as propagules and grown to 14 days under photosynthetic photon flux of 140, 200 and 320μ mol·m^-2·s^-1 during 1 to 5, 6 to 10 and 11 to 14 days after planting, respectively. Other environmental conditions included photoperiod of 16 h·d-¹, air temperature of 29°C, and CO₂ concentration of 920μmol·mo1^-1. At the planting densities of 59, 118, 236 and 473 m^-2, M were 5.9, 6.4, 3.8 and 2.3, respectively, and E_{p, 1} were 0.41, 0.19, 0.16 and 0.13 kWh (electricity cost : 6.2, 2.9, 2.4 and 2.0 Yen), respectively. Results suggest that production can be optimized and electricity cost can be reduced by manipulating planting densities.