Abstract：In order to study the influence of directional motion of nano-particles on dynamic light scattering measurements, we measured the nano-particles with different velocity using the measurement system of dynamic light scattering. The results show that the measured auto-correlation function is almost invariant when the directional velocity is perpendicular to the scattering wave vector. However, when the directional velocity and the scattering wave vector are not perpendicular, the decay of the auto-correlation function becomes faster as the directional velocity becomes larger. When the velocity is fixed, the angle between the velocity and the scattering vector is smaller and the decay of the function is faster. These results agree well with the theory of the dynamic light scattering when the waist of the incident light is limited.

Keywords：dynamic light scattering; Brownian motion; directional motion; nano-particle