The fatigue characteristic of TC4 titanium alloy by selective laser melting（ SLM） with two different surface roughness is investigated. The detail fatigue rating（ DFR） values are achieved based on the fatigue tests. The fatigue fractography is observed by scanning electron microscope to find the reason for surface roughness modification. The results indicate that DFR value of as-built surface specimen is 235. 1 MPa and machined surface specimen is 283. 3 MPa. So the SLM TC4 specimen in machined surface has superior fatigue property. The surface roughness has a significant effect on the crack initiation site. The fatigue crack in as-built specimen initiates from surface. Fatigue crack in machined specimen initiates from inside to subsurface.The latter is flat and the former is relatively rough.