Studying about fatigue in RC beams without stirrup is one of valid approaches to investigate fatigue problem in RC slabs because the transverse cracked slab can be considered as plural RC beams lined up in parallel. Therefore, in this paper, authors tried to clarify the fatigue damage in RC beams keeping the problem of RC slabs in mind. Non-linear three-dimensional finite element analysis (3D-FEA) of RC beams without stirrup is conducted under three types of loading conditions; static load, fatigue load and variable amplitude fatigue load. By comparing static and fatigue analysis results, authors examine the principal compressive stress distribution and the stiffness reduction. This paper presents that static and fatigue loading showed different trends in both the stress distribution and stiffness reduction. It can be also revealed that when variable amplitude load with different two load blocks is given in fatigue analysis, the cyclic loading in the first load block influences on the decreasing rate of stiffness.