Background: One of the medication attempts in diabetes mellitus is by utilising plants that are potent as an antioxidant. Eurycoma longifolia Jack. known as "Longjack" in English, is a medicinal plant and reportedly effective as an antioxidant. Objectives: This study was aimed to examine the antidiabetic effectiveness of ethanol extract of longjack leaf in diabetes mellitus rats. Methods: This study used the total of 24 male white rats which were grouped into four. The normal and the negative control groups were administrated with CMC-Na 0.5% dose 2 ml/200 g body weight; one group was administrated with ethanol extract of longjack leaf dose 176.4 mg/200 g body weight; and the positive control group was administrated with glibenclamide dose 0.09 mg/200 g body weight. Before the extract administration, all of the experimental animals were prior induced into diabetic condition with streptozotocin-nicotinamide. Results: The levels of blood glucose and malondialdehyde in rats after the 14-day extract treatments were 78.73 mg/dl and 1.13 nmol/ml, respectively in the normal control group; 285.84 mg/dl and 10.03 nmol/ml were in the negative control group; 156.77 mg/dl and 3.86 nmol/ml in the group with the administration of ethanol extract of longjack leaf; and 148.63 mg/dl dan 3.64 nmol/ml in the group of glibenclamide administration (p<0.05). The reduction of blood glucose and malondialdehyde levels in the groups of ethanol extract administration of longjack leaf dose 176.4 mg/200 g body weight was similar to the glibenclamide administration dose 0.09 mg/200 g body weight. Conclusion: The ethanol extract of longjack leaf was effective as an antidiabetic.