Tuberous roots of yacon (Polymnia sonchifolia) contain a large amount of oligofructans. They have a growing interested as a food for health and as a raw material of oligofructans. Then it is necessary to establish yacon culivation technique.We tested on the effect of the weight of seed tuber on the growth and yield of yacon.Three experimental plots were made; 10g plot, 30g plot and 50g plot. Three replications were made on each plot.The results were as follows:1) Growth of yacon in all stage tended to the best in 50g seed tubers planting and slow in 10g seed tubers planting.2) The number of stemes per a plant on November 2 were 4.3 in 10g plot, 5.1 in 30g plot and 5.9 in 50g plot, and it was significance in each plot.3) The top fresh weight was produced 967g/plant in 10g plot, 960g/plant in 30g plot and 1005g/plant in 50g plot, but it was nonsignificance in each plot.4) The yield of yacon tuberous roots in the weight was produced 1545g/plant in 10g plot, 1706g/plant in 30g plot and 1712g/plant in 50g plot, and the weight in 50g plot was closed to 30g plot, but it was nonsignificane in each plot.5) From the yield of yacon tuberous roots in each plot, it was suggested that the weight of seed tuber in planting would be desirable to 30-50g.