Carbon nanoparticles (CNPs) were prepared by simply mixing candle soot into ethanolic Nafion solution under ultrasonic radiation for 90 min. Well-defined CNPs could be obtained after centrifugation and washing. The products were characterized with transmission electron microscopy (TEM), X-ray diffractometry (XRD), and scanning electron microscopy (SEM). A possible formation mechanism is proposed. Electrochemical measurements show that they may be useful as biosensors.