Polyacrylonitrile(PAN) microspheres were synthesized by soapless emulsion polymerization of acrylonitrile(AN) in water. The effects of monomer and initiator concentration on the conversion of AN and the diameter of microspheres were investigated. The results showed that when the concentration of monomer increases, the conversion of AN decreases while the diameter of PAN microspheres increases. When the concentration of initiator increases, the conversion of AN and the diameter of PAN microspheres both increase. For this method of PAN microspheres synthesis, the highest conversion of AN is about 60% and the diameter can be controlled in need ranging from approximately 140 to 500nm.