Silica particles coated with PA12 by emulsion polymerization were used as fillers to reinforce PA12 based composites prepared by selective laser sintering (SLS). The influences of the treated and untreated particles on the sintering behavior and mechanical properties of the laser sintered specimens were investigated. It was found that there were many uneven holes in the untreated composites. However, for the treated composites, due to the silica particle surfaces treated by emulsion polymerization, the absorbance of laser was improved and the particles dispersed well in the polymer matrix; a full dense structure was obtained and the properties were enhanced, such as the tend strength increased 30%, the maximum value was 34MPa; the tensile strength increased up to 125%, the maximum value was 44.2 MPa, comparing to the unfilled PA12. Drawing from the results, it can be confirmed that a full dense structure can be obtained and the PA12 matrix was strengthened and toughened when the silica particles were coated with PA12 by emulsion polymerization.