Abstract: According to two-phase hydrodynamic theories, the motionof the aerosol particles in a Negretti sample head is analyzed in this paper.The motion trajectory of the aerosol particles during the sampling ofPM_10 and PM_2.5 is computed.The influence of various factors such as airflow flux, diameter of aerosolparticles on the movement trajectory is discussed. The results show thatthe two-phase hydrodynamics is effective in simulating the motionof aerosol particles within an impactor. It can offer a powerfulhelp for the design and manufacture of impactor.