We have developed a simple, one-step hydrothermal method for the synthesis of highly fluorescent carbon nanoparticles (F-CNPs) with a high quantum yield (68%) and good photostability. The method requires less reaction time and a lower reaction temperature as compared with the previous reported methods. The as-prepared F-CNPs exhibit excellent emission property and high stability, as well as excitation-independent emission behavior. Moreover, it is attractive that F-CNPs can be used as an effective fluorescent probe for the detection of mercury ions with good selectivity and sensitivity in an aqueous solution.