Warfarin is a high-alert medication, which may result in bleeding if used improperly. In our case, one elderly female with atrial fibrillation had taken warfarin for more than half a year, and her international normalized ratio (INR) was maintained within the therapeutic range. The patient began to take tramadol to alleviate her shoulder pain. Three days later she presented hematuresis and had ecchymosis in her right upper arm, and in the meantime her INR rose to 10.04. Clinical pharmacists analyzed the cause for bleeding by searching relevant literature, and finally discovered the interaction between warfarin and tramadol. On the basis of that, the clinical pharmacists provided pharmaceutical care, offered specific medication education, as well as assisted the physicians to establish the medication plan for warfarin reuse. Eventually, her INR declined to reference ranges, and her hematuresis and ecchymosis were alleviated significantly. This successful case reveals that clinical pharmacy services contribute to better treatment outcomes. Clinical pharmacists can play an active role in anticoagulation management in healthcare team.