The influence of building direction (0°, 45° and 90°) on high cycle fatigue properties of a selective laser melted Al-Si alloy was investigated through uniaxial loading fatigue tests under stress ratio of 0 in combination with multi-dimensional imaging and electron backscattered diffraction analysis. Surface/subsurface failures were induced by single irregular lack of fusion defect originated from un-melt powder or inhomogeneous microstructure. Fatigue properties decreased as building angle arose, attributing to the size difference of original defect dominating crack nucleation, and the distribution difference of melting pool boundaries deflecting crack growth. According to the stress intensity factor evaluation, the threshold value of long crack growth and the transition size of crack growth exhibited decreasing tendency as building angle increased. Considering the influence of building direction on defect size distribution, a fatigue index parameter controlled crack nucleation life prediction model was established and the prediction result was satisfactory for safety design.