Some part of biochemical energy is converted to mechanical energy depending on type of exercise and athletic status, and remaining part is released into the environment as heat. Air temperature, radiant temperature, relative humidity and air flow velocity are four main factors affecting the heat transfer mechanism of the human body [1]. For investigation of thermal behaviour of human body under different exercise loads, this study was performed in an air conditioned test room within the School of Physical Education and Sports of the Ege University. During the exercise under different loadings, thermal behaviours of athletes were obtained by the measurement of the core and surface temperatures depend on time. Therefore, 11 athletes who achieved the criteria for inclusion were chosen. In this test room, a cycle ergometer was used. These athletes were subjected to an experimental test which consists of a total four sessions. These sessions were 2 familiarization session, 1 aerobic power (VO2peak) test session and submaximal VO2 exercise test sessions. The athletes’ oxygen consumption and carbondioxide production and respiratory exchange ratios (RQ) were controlled by using gas analyzer on a 2-second time intervals regularly. The parameters such as heart rate exchange and fatique level were also monitored. In addition, the body temperatures different surface were determined by using thermal camera and core temperature recorded by using the core body temperature capsules periodically. During the cycle exercise, athletes’ total energy production was measured by gas analyzer; mechanical energy data from cycle ergometer was also calculated. The changes of the body thermal behaviours were calculated by the subtraction of the mechanical energy from the total energy. The result of this study indicate that while approximately one-fourth of the total energy produced by metabolism was used for performed the mechanical work during an exercise which intensity of VO2peak, the other part of the energy was balanced by means of sensible and latent heat transfer and thermal storage of the body depends on the body temperature exchange. Core temperatures more quickly increased observed in VO2peak test than %60 VO2 test.