Kayea assamica (King and Prain) is a native tree of Assam, India, which is known as Sia-Nahar in local language (Assamese). The aqueous extract of bark possesses antimalarial activity and methanolic extract of barks possess coumarin derivatives like therapin A, B and C, which are reported to have cytotoxic activity. This research is focused on the antioxidant activity and antidiabetic property of the leaves of K. assamica. Methanol, chloroform and hexane extract is used to estimate the polyphenol content. The methanolic extract showed highest phenolic content [(88.03 ± 0.33) mg GAE/g of dry material]. The flavonoid content [(58.44 ± 0.23) mg QE/g of dry material] in the methanolic extract was also highest. The antioxidant capacity was determined by DPPH free radical scavenging assay. The methanol extract was showing comparable IC50 value of 5.4 ± 0.54 μg/ml against the standard drug (ascorbic acid). The leaves extracts were tested for capacity to scavenge the ABTS free radicals. The capacity to quench the free radicals was highest in methanolic extract (IC50 value of 6.9 ± 0.54 μg/ml). The anti-diabetic capacity was investigated with the assist of α-amylase inhibiting assay and α-glucosidase inhibiting assay. The methanolic extract of leaves of K. assamica showed better α-amylase inhibition (17.42 ± 0.25 μg/ml) and α-glucosidase inhibition (IC50 of 13.19 ± 0.64 μg/ml) and was comparable to standard drug acarbose (IC50 of 19.42 ± 0.86 μg/ml). The result obtained can provide an insight into the utilization of leaves of K. assamica as potent antioxidant and anti-diabetic herbal drug.