The harvest object of a sweet sorghum plant is all parts of shoot which mainly consists of a juicy main stem, leaf blades and leaf sheaths. To obtain higher yield, it is important to understand in detail the yield production process. However, little has been known about plant age for sweet sorghum, which is an important parameter to predict the growth patterns of elongated internode (IN : defined as that having≥1.0cm), leaf blade (LB) and leaf sheath (LS). Thus, here we developed a new index of plant age (AL) by using the position of a fully expanded leaf on the stem. This study was conducted to clarify the elongation patterns of IN, LB and LS. Two cultivars, Syrup sorgo 2 (S2, a late-maturing) and Hybrid sorgo (HS, a medium-maturing), were gorwn under the field condition in 1992. A conceptional growth curve of IN, LB and LS was drawn by plotting their percentages of elongation on AL. IN (n) (n : natural number) began to elongate at AL(n+1), rapidly elongated at AL(n+2) and reached its maximum length at AL(n+3)∼AL(n+4). LB(n) rapidly elongated at AL(n-4)∼AL(n-2) and reached its maximum length at AL(n-1). LS (n) rapidly elongated at AL(n-1), reached its maximum length at AL(n)∼AL(n+1). Thus, AL can serve as an effective means of identifying the stage of IN, LB and LS development during the internode elongation stage.