An outdoor unit of air conditioner for ensuring strength against an impact applied to an electrical component box in a front-back, left-right, or top-bottom direction. The electrical component box has an upper partition plate, a front plate, and a box base plate coupled to one another and supported by an arm and a support post. The arm substantially perpendicularly supports the front plate in a front-back direction, and the upper partition plate. Thus, the electrical component box is strong enough to withstand an impact in the front-back and the left-right directions. Further, the support post substantially perpendicularly supports the arm and the box base plate in a top-bottom direction allowing the electrical component box to withstand an impact in the top-bottom direction. Also material of the electrical component box can be reduced.