This study was conducted to determine the optimal planting distance in cultivation of yacon (Polymnia sonchifolia Poeppig ＆ Endlicher). Plug seedlings were planted with 6 different plant densities of 80 $ imes$ 60, 80 $ imes$ 50, 70 $ imes$ 50, 70 $ imes$ 45, 70 $ imes$ 40 and 60 $ imes$ 45 cm. The plant height and the petiole length were increased with increasing the planting distance. The tallest plant height of yacon was 165.4cm with the plant density of 80 $ imes$ 60 cm. However, branch number per plant, leaf number on main stem and stem diameter were not significantly difference among planting densities. Tuberous root was harvested 31.42 tons/ha in 70 $ imes$ 50 cm spacing. The ratio of heavier tuberous roots than 200 g to total tuberous roots decreased significantly according to increase of planting density. Fresh weights of shoot and root, contain the crown bud, were decreased, as planting distance was shorter. Tuberous root number was fewer but its weight was heavier in wide planting than in dense planting. We think that optimal planting density is about 30,000 plants/ha, if it were to be 70 cm row spacing, intrarow spacing should calculate about 47cm.