Changes in chemical composition (vitamin C, polyphenol) and antioxidant activity of guava fruits collected in Dong Du, Gia Lam, Hano at different maturity stages were examined. The maturity stages were 'mature green, color turning, ripe and overripe. The results showed that vitamin C content sharply increased during ripening and reached the maximum value at ripe stage. Both polyphenol content and antioxidant activity decreased during ripening. Changes in polyphenol content was significantly correlated with changes in antioxidant activity.