Acute pancreatitis is the major complication of endoscopic retrograde cholangiopancreatography (ERCP). A preliminary research suggested that the administration of nonsteroidal anti-inflammatory drugs (NSAIDs) with nitroglycerin might reduce the incidence of post-ERCP pancreatitis (PEP) more effectively than NSAIDs alone. We conduct a two-arm, multicenter, prospective, randomized, superiority trial to evaluate the additional effect of nitroglycerin for prevention of PEP. A total of 900 patients randomly receive 50 mg diclofenac suppository either alone or with 5 mg isosorbide dinitrate sublingual tablet. The primary endpoint is the occurrence of PEP. This study will clarify whether NSAIDs plus nitroglycerin can prevent PEP.