Friction stir welding is a solid state welding method which appeared as a relatively new welding technology to be used mostly in aluminum alloys. The fatigue behaviours of welded joints represents the main problem for their industrial applications. In this research, fully reversed uniaxial fatigue tests have been performed in order to investigate the fatigue behaviours of single-sided friction stir butt welds in different welding conditions in 3 mm thick 3003-O non-heattreatable aluminum alloys. The employed rotating speeds of the tool were 1070, 1520 and 2140 rpm while the welding speeds were 40, 80 and 112 mm/min. The microstructure of the friction stir welds was studied by employing optical microscopy. The friction stir welds consisted of following microstructures: stirred zone, thermo-mechanically affected zone, heat affected zone and base material. The comparative studies on the fatigue behaviours between the base material and friction stir welds in different welding conditions have been done in this study. The fracture surfaces of the fatigue specimens were observed with a scanning electron microscope. According to the results, welding parameters have a major influence on the fatigue behaviours of the 3003-O friction stir welds. The fatigue lifes of FS welds with the welding speed of 40 mm/min at different rotating speeds are about 2-3 times longer than those of FS welds with the welding speeds of 80 mm/min and 112 mm/min at different rotating speeds at a fixed stress amplitude under the stress ratio R = -1. At a significantly lower welding speed and a higher rotatinal speed the fatigue life of the friction stir welds of 3003-O aluminum alloys was improved due to the increased amount of heat supplied to the weld per unit length. However, the fatigue lifes of all friction stir welds are lower than that of the base material at all stress amplitudes.