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Titanium alloys have many advanced applications where they are subject to fatigue. Here, we compare the fatigue crack growth resistance and microstructures of TC4-DT alloys fabricated by wire-arc additive manufacturing (WAAM; sample S1) and in-situ rolled WAAM (sample S2) followed by quasi-β heat treatment. Sample S2 had superior fatigue crack growth resistance, which is mainly attributed to its finer defects and finer α phase. The critical size of the defects related to fatigue performance was calculated. The majority of defects in sample S2 were smaller than the critical size, while several coarse defects (∼1.91 mm) were observed in sample S1. Multiple secondary cracks could be detected near the fracture surface in sample S2, leading to long and tortuous fatigue crack propagation paths. Therefore, in-situ rolling can improve the fatigue properties of titanium alloys fabricated by additive manufacturing.