Wind turbines exchange wake effects with each other on wind farms, resulting in wake loss that reduces the power production of the wind farm. Various wake models have been developed to calculate these wake effects. Among them, the Jensen wake model is the most widely used. The model has a wake decay constant parameter for considering the topographical environment of the wind farm. However, setting this wake decay constant relies on the subjective judgment of the designer. In this study, a simulation program was developed to examine the effect of the wake decay constant of the Jensen wake model. The simulation results were compared with data collected from an actual wind farm. Consequently, the output of the downstream wind turbine was reduced due to the wake. By analyzing the effect of the wake decay constant, when the recommended value was applied, the results matched the measured values in certain cases but did not in others. However, when the wake decay constant calculated using the turbulence intensity was applied, the results were generally similar to those of the actual data.