An apparatus and method of detecting a thermal runaway of a battery for an electric vehicle are provided. The apparatus includes a plurality of battery cells that are connected in series through a high voltage (HV) line and a battery module including the plurality of battery cells. A plurality of main sensing lines are electrically connected to the HV line between the plurality of battery cells to measure voltages of respective battery cells. A plurality of auxiliary sensing lines are respectively connected to an HV line as an input line of the battery module and to an HV line as an output line of the battery module to measure a voltage of the battery module.