Current standard therapy of primary biliary cholangitis-autoimmune hepatitis overlap syndrome(PBC-AIH overlap) consists of a combination of prednisolone and azathioprine. However, a significant proportion of patients may do not respond to, or is intolerant for azathioprine. Several studies have documented the efficacy and safety of mycophenolate mofetil(MMF) as second-line therapy for PBC-AIH overlap. However, robust evidence from a formal randomized clinical trial for the first-line immunosuppressor is in need.

Inclusion Criteria:

1. Patients aged 18-70 years;
2. Diagnosed with PBC-AIH overlap syndrome according to Paris criteria;
3. Agreed to participate in the trial, and assigned informed consent;
4. The WBC count ≥2.5x10^9/L and platelet count ≥50x10^9/L.

Exclusion Criteria:

1. The presence of hepatitis A, B, C, D, or E virus infection;
2. Patients with presence of serious decompensated cirrhosis;
3. Patients have a history of glucocorticoid or immunosuppressant medication before enrollment;
4. Liver damage caused by other reasons: such as primary sclerosing cholangitis, non-alcoholic steatohepatitis, drug induced liver disease or Wilson disease.
5. Pregnant and breeding women;
6. Severe disorders of other vital organs, such as severe heart failure, cancer;
7. Parenteral administration of blood or blood products within 6 months before screening;
8. Recent treatment with drugs having known liver toxicity;
9. Taken part in other clinic trials within 6 months before enrollment.