An outdoor unit of an air conditioner comprising: a partition plate provided so as to extend upright from a bottom plate of the outdoor unit to halfway to a top thereof; an electric component box placed on the partition plate and configured to divide the interior of the outdoor unit into a machinery chamber and a heat exchanger chamber with the partition plate; an electronic substrate stored in the electric component box and having a power module mounted thereon; a compressor provided in the machinery chamber; a propeller fan driven by a fan motor and a heat exchanger, both provided in the heat exchanger chamber; and an electric component partition plate being in contact at one end thereof with an end portion of the heat exchanger positioned on the side of a back surface of the heat exchanger chamber and at the other end thereof with a front plate of the outdoor unit, and configured to divide the interior of the electric component box and the heat exchanger chamber, wherein the electric component partition plate is inclined so that an extended line thereof toward a front surface of the outdoor unit intersects a line of projection of a front edge of a vane of the propeller fan, the electronic substrate is fixed to an opening provided on the electric component partition plate so as to be positioned within the electric component box and a heat sink for cooling the power module is fixed in parallel to a surface of the electric component partition plate in contact with the back side of the electronic substrate, and a plurality of heat radiation fins are provided on one surface of the heat sink so as to project toward the heat exchanger chamber side and extend in parallel to a flow of air on the electric component partition plate, characterized in that the heat sink is provided with a wind direction guide that covers the heat radiation fins attached thereto; and the wind direction guide includes a horn-shaped inlet member having an inlet port widening toward the heat exchanger on the side of the back surface of the heat exchanger chamber and a lid portion covering the section from an end on the side of the heat sink of the inlet member to an end on the side of an outlet port of the heat sink, while keeping constant space with respect to the heat radiation fins. a partition plate provided so as to extend upright from a bottom plate of the outdoor unit to halfway to a top thereof; an electric component box placed on the partition plate and configured to divide the interior of the outdoor unit into a machinery chamber and a heat exchanger chamber with the partition plate; an electronic substrate stored in the electric component box and having a power module mounted thereon; a compressor provided in the machinery chamber; a propeller fan driven by a fan motor and a heat exchanger, both provided in the heat exchanger chamber; and an electric component partition plate being in contact at one end thereof with an end portion of the heat exchanger positioned on the side of a back surface of the heat exchanger chamber and at the other end thereof with a front plate of the outdoor unit, and configured to divide the interior of the electric component box and the heat exchanger chamber, wherein the electric component partition plate is inclined so that an extended line thereof toward a front surface of the outdoor unit intersects a line of projection of a front edge of a vane of the propeller fan, the electronic substrate is fixed to an opening provided on the electric component partition plate so as to be positioned within the electric component box and a heat sink for cooling the power module is fixed in parallel to a surface of the electric component partition plate in contact with the back side of the electronic substrate, and a plurality of heat radiation fins are provided on one surface of the heat sink so as to project toward the heat exchanger chamber side and extend in parallel to a flow of air on the electric component partition plate, characterized in that the heat sink is provided with a wind direction guide that covers the heat radiation fins attached thereto; and the wind direction guide includes a horn-shaped inlet member having an inlet port widening toward the heat exchanger on the side of the back surface of the heat exchanger chamber and a lid portion covering the section from an end on the side of the heat sink of the inlet member to an end on the side of an outlet port of the heat sink, while keeping constant space with respect to the heat radiation fins.