Open spaces are an integral part of building design. One of the most important principles of designing open spaces is to pay attention to thermal comfort. In the meantime, educational buildings, due to the use of numerous open spaces, require special attention to achieve appropriate thermal conditions. The central courtyard form is one of the most common educational forms in Tehran. This research examines the effect of the dimensions of the courtyard on outdoor thermal comfort (with constant area) with the UTCI index. Therefore, simulation and optimization were done to find the best and worst dimensions in terms of the average percentage of time that spaces are within outdoor thermal comfort range, among 76 different dimensions of the yard with a fixed area. The results show that the best dimensions of the courtyard are approximately 8.6×11.6m2 which is a rectangular form with north-south extension. The difference between the best and the worst dimensions of the courtyard, in terms of outdoor thermal comfort, is 0.007°C and the difference in the percentage of time with outdoor thermal comfort is 0.018%. Therefore, by only changing the dimensions of the courtyard (other variables such as area remain constant), a very slight improvement in outdoor thermal comfort occurs inside the courtyard. Also, the measurements show that by moving from north to south in the courtyard, outdoor thermal comfort decreases significantly as you move towards the south of the site.