Fifty-day-old plug seedlings of yacon (Polymnia sonchiflolia Poeppig ＆ Endlicher) were transplanted on Mar. 23, Apr,8, Apr,23, May 8, May 23, and June 7,2002 at Jeju to evaluate the effect of transplanting date on the growth and yield of yacon in Jeju island. Delaying the transplanting until May 23 greatly decreased plant height stem height, main stem diameter and number of nodes per plant. The number of marketable tuberous roots was about 6.0 per plant with the early transplanting from March 23 to April 231 but decreased to 1.9 per plant with the latest transplanting time, June 7. The yield of marketable tuberous roots of early transplanted (March 23 to April 23) yacon was about 27 MT/ha, and was 32.4 MT/ha at April 23 transplanting, but was remarkably reduced at the three later transplantings, May 8, May 23, and June 7. Dry matter and soluble solid contents of marketable tuberous roots were 14％ and 15％, respectively, regardless of transplanting date. The weight of a marketable tuberous root ranged from 153 to 183.9 g at the first three transplantings, but gyeatly reduced with delaying transplanting until May 7. The optimum transplanting date for yacon in Jeju island seems to be from mid-April to late April considering tuberous root yields, the risk of late-spring frost, and cost of seedling raising.