TC4 alloy components with complicated geometries can be directly fabricated using selective laser melting (SLM) at a low cost. These components are often used under complex service conditions. Thus, it is important to investigate the effects of the stress ratio (R) on the fatigue crack growth (FCG) rate (da / dN) in SLM TC4 alloys with defects at the steady state to develop guidelines for damage-tolerance design and fatigue life assessment. In this work, SLM TC4 alloys containing two different microdefects were used to qualitatively examine the effect of the defect size on the da / dN at the steady state. In addition, comparative studies using an alloy with smaller defect sizes were performed at the steady state and at R = 0.1, 0.3, and 0.5. The relationship between the da / dN and stress intensity factor range(ΔK)was plotted and analyzed by fitting the Paris formula. The results show that the Paris formula parameter, m, is constant and the parameter, C, increases, which means that the increase in the defect size increases the da / dN. The da / dN increases with an increase in R, and the da / dN curves converge at low ΔK, which are reflected in the increase in the parameter, m, and the decrease in the parameter C. Additionally, there is a linear relationship between m and lgC (the common logarithm of C), which is not affected by R. Finally, the change patterns in the da / dN caused by the microdefects and R were analyzed along with the fatigue damage mechanisms.