The invention relates to an apparatus and method for diagnosing battery cell defects. The apparatus for diagnosing a battery cell defect includes a battery having a plurality of battery cells, a sensing portion configured to sense the battery to generate sensing information, a calculation portion configured to calculate real used power and a cell voltage of each of the plurality battery cells using the generated sensing information, a monitoring portion configured to store information on whether the battery is overused in the form of a power monitoring set using the calculated real used power, and a determination portion configured to determine, when the calculated cell voltage reaches a preset lower limit voltage value, whether each of the plurality battery cells fails using the power monitoring set and a cell voltage deviation.