Disclosed are an apparatus and a method for accurately estimating a state of charge of a battery, which can measure a change of temperature and an open circuit voltage so as to estimate the state of charge at an initial time when a vehicle is not driven, and while measuring a decrement in a capacity of a battery according to charging and discharging of the battery when the vehicle is driven. The method includes the steps of: measuring a temperature in an initial estimation of the state of charge; measuring an open circuit voltage; obtaining parameters indicating a change of the open circuit voltage according to a change of temperature; and calculating the state of charge using the parameters and the open circuit voltage which is measured depending on the obtained parameters. The method further includes the steps of: measuring electric current in order to integrate the electric current during an estimation of the state of charge after initial time; calculating a decrement in capacity of the battery according to cycles; and estimating the state of charge by dividing a value, which is obtained by integrating electric current, by the decrement in the capacity of the battery according to the cycles.