Herein a facile method is developed for fabrication of polystyrene/silver (PS/Ag) composite particles based on colloid thermodynamics. In our system, once the silver nanoparticles are formed in the presence of PS microspheres, they will play the role of steric stabilizer and attach spontaneously onto the surface of PS microspheres to reduce the Gibbs free energy of the colloidal system. The resultant PS/Ag composite particles have been characterized extensively by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and energy dispersive X-ray spectroscopy. The results indicate that the structure and morphology of the PS/Ag composite particles could be controlled easily by changing the molar ratio of sodium borohydride and silver nitrate. More significantly, the PS/Ag composite particles exhibit good catalytic activity and reusability at low catalyst concentration during the reduction of p-nitrophenol by sodium borohydride. Hence, it is expected that the as-prepared PS/Ag composite particles show promise and great potential for practical application in catalysis.