Nowadays, the baseboard heating systems have attracted the attention of many HVAC engineers because of its uniform temperature distribution and low feed water temperature. Despite this, the uniformity of indoor thermal conditions can be disturbed by some parameters such as exterior walls and air infiltration from window gaps. Therefore, the main goal of this study is to investigate the effects of air infiltration from window gaps on the performance of baseboard system and occupants’ thermal conditions. For this reason, a room has been considered under the terms of “ASHRAE-140 standard/Case 600” and climatic conditions of Tehran with winter outdoor design temperature of -10C. Also, the heat power on the baseboard panel has been set as much as the average of occupants’ thermal dissatisfaction index stays within the allowable range (lower than 10%). The results show that the heating baseboard system can provide the appropriate thermal conditions for sitting occupants with average panel temperature of 43C. In spite of this, the distribution of occupants’ dissatisfaction index near the floor is not uniform. The results indicate that the air infiltration can cause to increase the thermal dissatisfaction index up to 40% in the floor region.