The investigators intends to carry out a randomized, cross-over, prospective study which will last 48 weeks in youth with T1DM followed up by the center. The purpose is to observe the effects of metformin on glycemic control and insulin sensitivity in adolescents with T1DM based on insulin therapy by using 72h CGMS and hyperinsulinemic euglycemic clamps.

Inclusion Criteria:

1. Type 1 diabetes mellitus;
2. Age≥12 yr;
3. Tanner sexual maturation rating 2-5;
4. HbA1c levels: 7.5%-10%;
5. MDI or CSII ≥6 months;
6. Insulin dose ≥ 0.8U/kg before the enrollment，and the dosage is stable at least >1 month（dosage change<10%）.

Exclusion Criteria:

1. Serious acute and chronic complications associated with diabetes;
2. Repeated and serious hypoglycemia episodes repeatedly;
3. Hepatic function damage (ALT≥2.5 times higher than the upper limit of the normal accepted range);
4. Moderate to advanced renal impairment (calculated according to MDRD equation: eGFR<60ml/min/1.73m2);
5. Clinically significant heart attacks: myocardial infarction, arrhythmia，II-III°AVB，unstable angina，decompensated HF（NYHA III~IV;
6. Female patients who have a sex life and are not willing to use contraceptive methods;
7. Pregnancy.