Abstract: In this paper,the authors use Friction-type miller N -70 as a test miller,and BRP-3/70 type pressure sensor and BTY type Multiple-tester to measure its chamber pressure . Factors forming and affecting the chamber' s pressure are analysed. Results indicate that the pressure fluctuates cyclically and the degree of fluctuation varies according to the rice varieties and the position in the chamber. The maximum pressure is 20%～35% greater than the average. Orthogonal test indicates that both the outlet pressure and the clearance between milling rool and screen affects the chamber' s pressure greatly. Shaft revolution scarcely affects the chamber' s pressure. The kernel breakage of round-grained rice and Indica rice increases along with the increase of pressure,while the amount of rough rice among milled rice decreases. The experimental results show that unhulled rice among the milled Indica rice decreases with the increase of kernel breakage and the authors suggest that it is appropriate to keep the outlet pressure at 45. 6kPa when milling round-grained rice and it is not appropriate for milling Indica rice.