In order to evaluate aerosol penetrations through a fully developed laminar flow in a cylindrical tube, the trajectories of aerosol particles were calculated by a Langevin dynamics equation that can represent the Brownian motion of aerosol particles. In our calculations, two criteria of penetration distance were employed: the maximum distance that a particle travels in the axial direction before it is deposited on inner wall of the tube, z(MTD); and the axial distance from the inlet of tube to the point of deposition, z(DD). The distributions of these two penetration distances enable us to evaluate respectively the classical penetration, defined as the ratio of total particle flux over a cross section of tube to the total particle flux at tube inlet, and the distribution of deposition flux to the tube wall. At high Pclet numbers of Pe > 1000, there is almost no difference between z(MTD) and z(DD). The resulting penetrations calculated from the distribution of z(MTD) agree well with the conventional analytical solution of convective-diffusion of aerosols in which the diffusion of aerosol particles in the direction of flow is neglected. At low Peclet numbers of Pe < 1000, the isotropic nature of Brownian motion of aerosol particles becomes obvious: the ratio of particles of which z(DD) is smaller than z(MTD) significantly increases. The distribution of z(MTD) successfully reproduces the results of aerosol penetration obtained by the numerical solutions of the convective-diffusion equation of aerosols without neglecting the diffusion in the direction of flow, but the deposition flux obtained by the distribution of z(DD) does not agree with the gradient of particle penetration, which is a numerical solution of convective-diffusion equation of aerosol particles. Consequently, it was concluded that the method of calculating the trajectories of particles directly was advantageous to evaluate the particle behavior at low Peclet numbers.