The invention provides a method for estimating a battery capacity of an electronic device. When a current or an average current of the battery is zero, a reference capacity is inquired from a look-up table based on a voltage of the battery. When a previously remaining capacity is greater than a reference capacity, a deduction process of a self-consumable capacity is performed one or more times for a previously recorded remaining capacity to obtain a new recorded remaining capacity smaller than the reference capacity. When the current of the battery is less than zero, a deduction process of a count capacity of discharging is performed one or more times for the previously recorded remaining capacity until a power of the battery is exhausted to obtain a first offset capacity. The first offset capacity is deducted from a previously recorded full charge capacity to estimate a new full charge capacity.