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The time-of-flight mass spectrum (TOFMS) relative to the resonant two-photon ionization of gas phase p-dichlorobenzene was obtained in the wavelength range of 240-250 nm by a home-made system. A special design was made to reduce the effect of memory on the inner wall of the sample inlet system suitable for the investigation of semi-volatile organic compound. In this wavelength range, p-dichlorobenzene molecules firstly absorbed one photon to be excited from the ground state 1Ag(S0) to the first excited state 1B2u(S1), then absorbed another photon to be ionized. The relationship between the signal intensity of p-dichlorobenzene molecular ion C6H435Cl+2 at 248-nm wavelength and the laser power was given. The 1.52 power index of C6H435Cl+2 was a typical identification of the 3/2 power law. The relationship between the ion signal intensity of C6H435Cl+2 and the sample concentration was close to a linear one in the ppm(V/V) range, which led to a detection limit of 125 ppb(V/V) for p-dichlorobenzene.