The invention provides an estimation method for state of charge (SOC) of battery. A reference SOC is inquired from a look-up table based on a voltage of battery detected. A voltage threshold is subtracted from the voltage of battery to obtain a voltage difference. A current SOC or the reference SOC is divided by the voltage difference to obtain a first slope or a second slope. The first slope is divided by the second slope to obtain an adjustment ratio. An electric charge capacity for charging the battery is counted by a coulomb counting method to obtain a count capacity of discharging that is multiplied by the adjustment ratio to obtain an adjustment value. The adjustment value is subtracted from a previously recorded remaining capacity to obtain a new remaining capacity. The new remaining capacity is divided by a full charge capacity to estimate a new SOC of battery.