Polystyrene/carbon nanotubes composites were prepared with the method of in situ polymerization. The mechanical properties of the composites were studied as well. Moreover, the relationship of the properties and the microstructure of the composites was investigated. The results showed that the addition of carbon nanotubes made tensile strength, toughness and breaking elongation of the materials increase. When the dose of carbon nanotubes was 1.0%, the composites had the best mechanical properties. Scanning electric microscope images displayed that carbon nanotubes dispersed in the matrix in na...