Fatigue design performance is an important factor for the industrial application of selective laser melting (SLM) technology. The SLM process is known to generate surface and internal defects, induced by material phase change during fabrication, which can lead to variability in fatigue performance of as-built, near-net-shaped specimens and parts. This paper investigates the fatigue behavior of as-built, machined, hot isostatic pressed (HIP) and shot peened specimens to assess the effects of these post-processing methods on fatigue life outcomes. SLM process vertically built 316L fatigue test specimens, in accordance with ASTM E466 hour-glass flat specimen geometry, were tested in the as-built condition, and the specimens were post-processed by machining, HIP treatment, and shot peening. The test results identified that the SLM fabricated materials exhibited improved fatigue properties through surface smoothing after the shot peening, resulting in a fatigue life increase of up to 65% compared to those that only underwent the HIP process. We also confirmed that in over 90% of the test specimens, the fatigue fracture occurred from the crack that initiated and propagated at the edges of the rectangular gauge section, which also supports the viewpoint that the surface defects act as weak points under the cyclic loadings. The findings primarily suggest that enhancing the fatigue properties of the additive manufacturing processed metal structures should focus on removing surface defects and improving the surface roughness, post-processing the surface condition of the fatigue test specimens.