The effects of Si content on densification and mechanical and thermal properties of Al-Si binary aluminum alloys processed by selective laser melting (SLM) were systematically studied. Densification of the SLM specimens with Al-0 and 4~20Si alloys was achieved by optimizing the laser scan parameters. The SLM specimen with Al-1Si alloy contained many microcracks which originated from solidification cracks as suggested by microstructural analyses. That is the Al-1Si SLM specimen in solid-liquid coexisting state was brittle and no healing of cracks by infiltration of the liquid phase occurred leading to the generation of microcracks due to thermally induced tensile stresses during solidification. With increasing Si content in the Al-Si SLM specimens the strength increased whereas elongation and thermal conductivity decreased. The changes in these properties could be explained by the characteristic microstructures of granular oxides (Al-0Si) cellular dendrites (Al-4~12Si) and petaloid β-Si/eutectic phases (Al-20Si).