Pure AlSi10Mg alloy and Carbon Nanotubes (CNTs)-AlSi10Mg composite with different CNTs addition were fabricated by selective laser melting (SLM). The CNTs-AlSi10Mg composite was strengthened when the CNTs content was lower than 0.05 wt.%. With the increasing CNTs content, however, the strength was decreased significantly because of the poor density. In order to understand the influence of CNTs on the defects in SLMed AlSi10Mg alloy, nano-CT technology was used to get the 3D information of the defects. The results indicated that the volume fraction of large defects(with diameter larger than 50 μm) in the total volume of defects was increased from 12% to 46% in CNTs(0.5wt.%)-AlSi10Mg composite. The number of gas pores in CNTs(0.5wt.%)-AlSi10Mg composite were significantly increased. The diameter of gas pores in CNTs(0.5wt.%)-AlSi10Mg composite were larger than that in pure AlSi10Mg alloy. The agglomeration of CNTs in the powder and gas adsorption are the fundamental reasons for the increase of the two types of defects.