Abstract: Aerosol mass spectrometer as a powerful and on-line device has been widely utilized to probe the chemical composition of aerosol particles in the atmosphere. Several kinds of interfaces to transfer particles from air into vacuum have been employed in aerosol mass spectrometers. Among them capillary was developed as an interface to deliver large particles with a limited size distribution before. We proposed here to use capillary as a sampling interface for nano-aerosol mass spectrometer and the penetration efficiency of nano-particles in capillary was investigated and presented. A home-made reactor was employed to generate nano-particles through X-ray ion-induced nucleation of gaseous mixture. The size distributions of the nano-particles at the inlet and the outlet of capillaries were measured, respectively, by using a commercial nanometer scanning mobility particle sizer (Nano-SMPS) and then the penetration efficiency was obtained. The size distribution of the particles generated from ion-induced nucleation was measured to be around 3-20 nm, where the lower detection limit was limited by the commercial Nano-SMPS. The penetration efficiency was dependent on the length and the inner diameter of capillary, in particular for the nano-particles with size of less than 10 nm due to the influences of Boltzmann diffusion et al. The penetration efficiency would decrease with the length and the bore size of the capillary as the particle diameter was less than 10 nm. The dependence of the penetration efficiency on the length and the bore size of capillary becomes insignificantly as the particle diameter is close to 20 nm. In addition, the penetration efficiency can be benefited from a large flow rate with a short transmission time in capillary. Then the penetration efficiency of the nano-particles in the capillaries was theoretically calculated by the aerosol transport and deposition model. The results showed that the obtained theoretical data could be comparable with the experimental results and their discrepancies were also discussed. As a representative example, a laser-based photoionization aerosol time-of-flight mass spectrometer (ATOF-MS) combined with the capillary interface was utilized to probe the chemical components of nano-particles and the initial results have also been presented in the article. These results have demonstrated that the capillary can be used as the sample interface of nano-aerosol mass spectrometer to deliver nano-particles. Also, on the basis of these results, it is of very interest to study the formation and evolution of atmospheric fine particles below 100 nm by nano-aerosol mass spectrometer in the future.