The transport of nanoparticles in the boundary layer is closely related to particle mixing or deposition. We present an in situ imaging of TiO2 particle volume fraction near gas stagnation plane in a counterflow flame by recently developed phase-selective laser-induced breakdown spectroscopy technique. The concentration boundary layer is well resolved with a spatial resolution of 10 pm. Together with a numerical analysis of particle transport equation, the roles of convection, diffusion and thermophoresis are discussed. The calculated profile of particle volume fraction agrees well with experimental measurements, which indicates that current model of nanoparticle transport is capable to quantitatively predict the concentration profile in boundary layers. Further study shows that altering thermophoretic velocity shifts the concentration boundary layer but does not change the shape of concentration profile. The decaying slope is mainly controlled by diffusion process that is dependent on particle size.