Abstract: Ultrafine nanoparticles and new particle formation are significant components of aerosol particles in the air, which can influence the climate change and the quality of life in many ways. To get their chemical compositions is very important and helpful to reveal their sources and then make appropriate decisions to control their emissions efficiently. However, ultrafine nanoparticles are extremely small within nanometer size range, it is a technical challenge to probe their chemical compositions. More concretely, the traditional aerodynamic lens popularly utilized in a great deal of aerosol mass spectrometers is not suitable to transmit and focus ultrafine nanoparticles from air into mass spectrometer with a high vacuum, due to the Brownian movement. So up to now the related analytical instruments and the data of their chemical compositions are still very scarce. In this article, a home-made capillary interface laser desorption ionization aerosol time-of-flight mass spectrometer was presented to analyze on-line probe chemical composition of ultrafine nanoparticles with diameters of less than 100 nm in laboratory was presented. A silicon capillary with an inner diameter of 0.7 mm and a length of 18 cm were adopted as atmospheric interface to focus and transfer nanoparticles into the aerosol mass spectrometer. A 266 nm Nd:YAG laser combined with a tight focusing circular lens and a softly focusing cylindrical lens were employed to desorb and ionize nanoparticles, and then ions were detected by a linear time-of-flight mass analyzer. At the downstream of the particle beam and below the laser focusing spot, a faraday cup was installed in the aerosol mass spectrometer to collect and measure the current of the charged ultrafine nanoparticles. A commercial scanning mobility particle sizers (SMPS) was used to measure the size distribution and the total concentration of ultrafine nanoparticles on-line. As representative examples, ultrafine nanoparticles were produced from ion-induced nucleation reaction of Air/SO₂/H₂O mixture gases or spray of KCl, NaCl and NaI/NaBr aqueous solutions with a home-made ion-induced nucleation reactor and a commercial aerosol generator, and then their chemical compositions were measured by the aerosol mass spectrometer. These experimental results also showed that ultrafine nanoparticles were easier to be ionized with elemental or atomic compositions than the traditional particles with a big diameter. But after changing the tight focusing circular lens with the softly focusing cylindrical lens, the molecular ions of the ultrafine nanoparticles could be probed in photo ionization mass spectra too. In addition, with the current and the total concentration of the charged ultrafine nanoparticles, the transmission efficiency of the capillary interface was tested and its value was measured to be 0.33%, in accordance well with the reported results of similar capillary interfaces to transfer molecules in air.