In the United States, 18% of total primary energy consumes in commercial buildings. Windows in commercial buildings are responsible for 35 and 28% of heating and cooling loads respectively and 32% of the total building energy consumption. Geometrical design parameters of the window have significant impact on the building energy consumption since they define the window dimensions which interact with the light rays. Aspect ratio, the ratio of the width to the length of the window, is one of the geometrical parameters which needs more investigation in terms of their impact on building heating and cooling loads and light energy consumption. The objective of this study is to determine the optimum aspect ratio of the window in small commercial buildings in order to make balance between heating and cooling loads and lighting energy use in seven different climate zones in the United States. Small commercial buildings (i.e., less than 50,000 ft2) are selected for this study because they represent 94% of U.S. commercial buildings. To achieve this objective, first, the Hill Climbing algorithm is used to develop an optimization model. The developed model is coupled with the Energy Plus software to simulate energy performance of the building. An office building is modeled as a case study and is simulated in seven different climate zones (climate zone 1-7) in the United States. Finally, the results are analyzed and compared with the baseline model to show the energy loads and lighting energy use of the buildings having optimum window aspect ratio. In addition, based on the simulation results, recommendations are made for the designers for each of the seven climate zones.