A steam iron includes a housing, a water tank and a boiler mounted in the housing, a water supply pipe for guiding water from the water tank to the boiler for boiling, a relief pipe connected between the boiler and the water tank for guiding steam from the boiler to the water tank when an internal pressure of the boiler is excessively high, a valve flap-operated one-way check valve mounted in the water supply pipe to let water pass from the water tank to the boiler and stop steam from pass through the water supply pipe to the water tank, and a piston-operated relief valve mounted in the relief pipe for enabling steam to pass to the water tank when the internal pressure of the boiler is excessively high.