Inclusion Criteria:  Meet the American College of Rheumatology (ACR) criteria for symptomatic knee osteoarthritis assessed by a rheumatologist; Age 50 to 75 years; Body mass index ≥ 24 kg/m2; Knee pain ≥ 20 on a 100 mm visual analogue scale (VAS); Able to listen, speak, read and understand Chinese; capable of understanding the study requirements and willing to cooperate with the study instructions; able to provide written, informed consent.  Exclusion Criteria:  Severe radiographic knee OA as grade 3 joint space narrowing according to the Osteoarthritis Research Society International (OARSI) atlas; Severe knee pain as knee pain ≥ 80 on a 100 mm VAS; Planned knee or hip surgery (including arthroscopy, joint replacement, and joint open operation) within 2 years; Severe knee trauma history (including arthroscopy, severe injury of knee ligament or meniscus); Contraindication to MRI scanning (e.g. implanted pacemaker, artificial metal valve or cornea, aneurysm clipping surgery, arterial dissection, metal foreign bodies in the eyeball, claustrophobia); Other forms of inflammatory arthritis (e.g. rheumatoid arthritis, psoriatic arthritis); Active malignant cancer or other life-threatening diseases; Type 1 or type 2 diabetes mellitus; Clinical manifestation of liver dysfunction or alanine aminotransferase/aspartate aminotransferase exceeding 2 times the upper limit of normal value; Estimated glomerular filtration rate less than 60 ml/min/1.73 m2; Hypoxic state (e.g. chronic heart insufficiency, acute myocardial infarction, heart failure, chronic obstructive pulmonary disease, cor pulmonale, peripheral vascular disease); Alcoholism; Pregnancy or lactation; Allergic to metformin hydrochloride; Conditions affecting the absorption of oral drugs (e.g. postgastrectomy and malabsorption syndrome); Use of metformin in recent 30 days or plan to use metformin in the next 2 years; Use of investigational drug in recent 30 days.