In order to find out hypoglycemic constituents from natural resources, Korean medicinal plants being used for the treatments of diabetes mellitus were tested with streptozotocin-induced diabetic rats. From the above screening, the $CH_2Cl_2$ fraction of 80% methanol extracts of Morus alba leaves showed hypoglycemic activity. The $CH_2Cl_2$fraction was chromatographed on silica gel, resulting in 9 subfractions, and tested hypoglycemic activity. The subfraction which had a strong hypoglycemic activity was rechromatographed and two galactolipids (compounds 1 and 2) were isolated along with a triterpene (compound 3) from other fractions. On the basis of spectroscopic data, acid hydrolysis, alkaline hydrolysis and enzyme hydrolysis results, compounds 1, 2 and 3 were identified as $1-O-(9Z,12Z,15Z-octatrienoyl)-2-O-(9Z,12Z,15Z-octatrienoyl)-3-O-{eta}-D-galactopyranosyl$ glycerol, $2(R)-1-O-octadecanoyl-2-O-(9Z,12Z,15Z-octatrienoyl)-3-O-{eta}-D-galactopyranosyl$ glycerol, and $3{eta}-hydroxylup-20(29)-ene$ acetate, respectively. Compound 2 (1mg/kg p.o.) has approximately 16% hypoglycemic activity in streptozotocin-induced diabetic rats.