The effect of photoperiodicity on the growth of lettuce was experimentally examined for lettuce using the red and blue light-emitting diodes (LED) by changing day and night photoperiods. The result showed that the growth in the case of photoperiod of 4 hours was the largest compared with 12- and 6-hour cases for both fresh and dry weights by more than 60% at the largest. The result may be understood as the photosynthetic promotion effect by the rapid removal of assimilates due to the translocation during the night period.