A charging mode control circuit (320) includes a detecting circuit (321) and a charging control circuit (322). The detecting circuit detects whether a power/data interface (310) is in a communication state, and accordingly outputting a control signal. The charging control circuit receives the control signal and obtaining power from the power/data interface to charge a secondary battery (330) with a charging mode in accordance with the control signal. The charging mode is a fast charging mode with a charging current larger than a predetermined level, or a slow charging mode with a charging current not larger than the predetermined level. A portable electronic device (30) incorporates the charging mode control circuit is also provided.