Abstract: Laser desorption and ionization positive and negative ion mass spsctra of benzoepgrene and fluoranthene molecules are investigated by 337 nm N2 laser and time of flight mass spsctrometer. Parent ion dominated mass spctra are obtained under laser intensity (2~10) ×106W/cm2 when a larger laser spot(35μm×165μm)is formed on the surface of solid sample. This condition is suitable for mixture analysis by laser desorption and ionization method. The relations of the intensity of parent ions on lasger energy, size of la8er spot on solid surface,ionization potential (IP) and electronic affinity(EA) are discussed.