Abstract: The scattering light intensity can be enhanced in Rayleigh scattering measurement velocity test by adding a small quantity of nano-particles into low density wind tunnel flow field. It is a key factor for the accuracy of measurement result whether nano-particles can adapt the variation of the flow velocity. To investigate the measurement velocity of nano-particle by Rayleigh scattering test whether or not can represent the local flow field velocity, a two-way coupling DSMC method used in rarefied two phase flow is applied to simulate following features of nano-particle in low density flow filed with large grads. TiO₂ particles with 10 nm, 50 nm, 100 nm diameter in low density hypersonic flows around a spaceship model in M6 and M12 cases are carried out respectively. It is shown that the following features of variety size nano-particles in different rarefication flow are distinguishing, and the following feature of smaller size nano-particle is good in complex hypersonic flow. In simulation results, 10 nm nano-particle's following feature in lower rarefication of M6 is better, and good agreement with Rayleigh scattering measurement result. Following features of particle with diameter larger than 50 nm are bad, and in higher rarefication of M12, even 10 nm nano-particle's following feature become worse. This means that velocity of particle measured by Rayleigh scattering can not reflect velocity of flow field.