Diabetes mellitus, commonly known as diabetes, comprises a group of metabolic disorders characterized by a high blood sugar level over a prolonged period of time. Diabetes is classified into type 1 diabetes and type 2 diabetes, and the incidence and prevalence of diabetes, mostly type 2, have increased remarkably in recent decades. A diagnosis of type 2 diabetes is greatly influenced by heredity, and it is important to prevent diabetes in people with a family history by improving lifestyle and environmental factors. Controlling overeating, obesity, lack of exercise, and stress is effective to prevent diabetes. The patient in this case report had impaired fasting glucose and mild hyperlipidemia. The patient experienced improvements in these sequelae after the administration of herbal medicine (Yangkyuksanwha-tang) for 12 weeks and the implementation of a plant-based diet. The complete blood count, XXXX, fasting blood sugar, HbA1c, insulin, and C-peptide levels were measured, and the Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) and HOMA-β were calculated according to the FBS and fasting insulin levels. Total cholesterol, triglyceride, FBS, and HbA1c levels all decreased after 12 weeks compared with baseline measures. There was no change in the insulin secretory ability; the C-peptide level did not change as a result of β-cell function; and the HOMA-β level reflected an improved insulin secretory ability.