The purpose of this study was to establish the chemical compositions, enzyme activity and nutritional values of four cultivars of yam tubers at different stages of their maturation, and compare such levels during yam growth. The results have indicated that yam tubers featured a reasonably substantial content of protein when compared with other root and tuber crops, the protein content being of the order of 10.4–13.0 g/100 g (dry basis (db)) at time of harvest (day 260 post-emergence). Starch content of the four yam tubers increased as growth progressed and remained in the range of 70.5–85.3 g/100 g (db) during their growth period. The activity of yam-contained polyphenol oxidase (PPO) decreased remarkably over the early period of harvest (day 155–225 post-emergence), and subsequently decreased only slightly as growth progressed to harvest for all tested species of yam with the exception of CH yam tubers. Contrasting this, however, the activity of α-amylase increased significantly over the growth period for all cultivars, and dioscorin content of yam tuber also increased as growth progressed for all cultivars. All yam cultivars contained remarkably substantial amounts of essential amino acids, all of which were superior to the FAO reference pattern for such amino acids except for sulfur-containing amino acids and lysine contents.