Disclosed is an anti-obesity active extract obtained from a medicinal plant. More specifically, the extract obtained from lysimachiae foenum-graeci herba (which is a medicinal plant traditionally used in the East) can be used as an active ingredient for a raw material, a functional food, cosmetic, a crude drug, etc. for an obesity preventing or therapeutic agent because the extract can suppress the adipocyte differentiation and decrease the body weight and body fat in an obese animal, thereby suppressing fat accumulation (that is, a factor for obesity).