Guava (Psidium guajava L.) has traditionally been used as a remedy for diabetes mellitus among people in the tropical and subtropical regions. To investigate the preventive effect of guava leaf tea (GLT) on the increase in blood sugar level, several studies were performed both in vitro and in vivo. An in vitro study showed some inhibitory effects of GLT on the activity of α-amylase, a group of starch-hydrolyzing enzymes. It was found that GLT markedly restrained the gastrointestinal transport and absorption of glucose in the small intestine after an oral administration of 1g of glucose to rats. GLT also suppressed the increase in blood sugar levels induced by the glucose administration. These results suggest that the inhibitory effect of GLT on the increase in blood sugar levels may be attributable to its inhibition of saccharolytic enzyme activity, as well as of the gastrointestinal transport and absorption of glucose in the small intestine.