Abstract: A new atmospheric pressure direct analysis ion source, desorption corona beam ionization(DCBI) source for mass spectrometer was developed. The corona discharge was formed by applying a high DC voltage on the sharply pointed tip of a stainless tube. A ring electrode which serves as the counter electrode was placed several millimeters away from the tip of tube. Certain inert gas was supplied through the stainless steel tube in aid of the corona beam generating. Helium is a good choice for creation of a luminous beam although the nitrogen works as well at lower efficiency. The luminous beam caused by corona discharge was used for desorption/ionization of analyte. The inert gas was heated by a controlled heater before entering the stainless steel tube and forming the corona beam. It was the thermo effect of the heated gas stream that caused the desorption of the analyte. The corona beam here is a kind of low temperature plasma which caused chemical ionization to the analyte in the gas phase. The visual beam formed at the tip of stainless steel tube extended out for 8 to 12 mm and went through the ring electrode. The sampling spot can be determined by the beam tip location during the experiment.A DCBI source was successfully mounted to a single quadrupole mass spectrometer(LCMS-2010EV, Shimadzu, Japan). 11 pesticides including triadimefon, fenvalerate, trichlorfon, fenitrothion, cypermethrin, N-cyhalothrin, fenpropathrin, deltamertrin, methamidophos, tau-fluvalinate, parathion-methyl were detected directly. All samples were dropped on a small ceramic plate which served as sample holder. The holder was placed at a fixed spatial location between DCBI and inlet of MS. 3 µL sample solution was added on the sample holder each time. The DCBI source Conditions are: He 0.3 L min^－1, temperature of DCBI heater 200-450 ℃, interface voltage ＋3.0 kV. With these parameters 11 pesticides were all ionized and detected by DCBI-MS either in single or in mixed sample condition. LOD of 1 mg L^－1(about 1-10 nanogram of each compound) was achieved. In concentration range of 10-1 000 mg L^－1, there was an obviously positive correlation between signal intension and sample concentration.