In this study, thermal-oil cycling process was applied to adhesively bonded glass fibre reinforced single lap composite joints (SLJs) in order to investigate the effects of thermal fatigue on their tensile properties. Joints were subjected to thermal cycling to achieve the thermal fatigue effect. Initially, separated specimens were heated on a magnetic stirrer from 25 degrees C to 80 degrees C, 25 degrees C to 100 degrees C and 25 degrees C to 120 degrees C and kept at these temperatures for 10 min. Right after that specimens were immersed into heat transfer oil cooled with dry ice at -10 degrees C for 10 min. and placed back into the magnetic stirrer to reach intended temperatures again. Thus, one cycle is completed. This thermal cycling process was repeated for 1, 10, 20, 30, 40 and 50 times for different groups of specimens. Single lap shear (SLS) tests have been performed for the determination of mechanical properties. As a result of this study, it is found that the load carrying capacities of specimens generally decrease as the thermal cycling count is increased. However, it is observed that specimens subjected to 30 times thermal cycling have the maximum load carrying capacity.