Bamboo forests have been expanding by replacing surrounding ecosystem recently, and the expansions of bamboo forests could alter local water cycling due to significantly large transpiration in bamboo forests. But the mechanism of bamboo forest transpiration is still not clearly understood. Bamboo forest might have significant year-to-year variations in age structure and DBH distribution compared with woody plantation. For better understanding the impact of the variations in stand structures on stand-scale transpiration, we should clarify effects of culm age on sap flux. This study conducted sap flux measurements in a Moso bamboo forest in National Taiwan University (NTU) Forest, in central Taiwan. This study aimed (1) to show the variation of the age structure and DBH distribution of the Moso bamboo forest for several years, (2) to quantify relationship between culm age ,individual sap flux and transpiration. In this study, the distribution of DBH and age structure showed year-to-year variation for three years. As well, the result showed that culm age effected on the sap flux and transpiration, that is, sap flux and transpiration in older culms were significantly lower than that of younger age culms with slightly different diurnal variations.