Fatigue damage occurs in steel joints exposed to repeated loads during their service life. Even if the design of lattice tubular structures that make up the steel skeletal structures is carried out according to certain regulations, the evaluations for determining the most critical fatigue damage in these structures are limited. Moreover, the discrete geometric configurations of trusses make it difficult to accurately predict fatigue damage. In this study, fatigue analyzes were performed on the planar T-joint sample with two different methods and under different boundary and loading conditions. Three-dimensional finite element models and three-dimensional welded joint detail were created to perform more accurate analysis. As a result, it has been determined that the fixed type support increases the fatigue life due to the increase in rigidity.