A method for determining performance of a battery for a mild hybrid electric vehicle may include detecting a speed of the mild hybrid electric vehicle, a position value of an accelerator pedal, a position value of a brake pedal, and a state of charge (SOC) of the battery, determining whether a regenerative braking condition is satisfied, determining a demand braking amount of a driver, determining a regenerative braking allowance, operating a mild hybrid starter and generator (MHSG) to generate regenerative braking torque corresponding to the regenerative braking allowance, determining a target SOC increment amount, determining an actual SOC increment amount, and determining that the performance of the battery is deteriorated when a difference in value between the target SOC increment amount and the actual SOC increment amount is equal to or greater than a reference value.