In this study, the effect of boronizing on the fatigue life of AISI 1040 steel was investigated. The boronizing process was carried out in the fluid bed furnace at temperatures 850 degrees C and 950 degrees C for 2, 4 and 6 hours. Boron layer thickness, micro structure, micro hardness and fatigue strength of the treated samples were investigated. The boronized layer on the surfaces became thicker by the increment of the process time and in addition micro hardness increased with the increment in process temperature and time. As a result of the fatigue tests conducted for comparison; shorter fatigue strength values were determined in boronized samples compared to cold drown AISI 1040 sample experiments. Samples which boronized at 950 degrees C for 6 hours had the lowest fatigue live under all experimental conditions and the highest fatigue cycle numbers were determined in non-boronized samples in fatigue tests.