Abstract: In order to study the motion of the mixture of corn straw particles and spherical ceramic particles in vertically mounted square pipe, a set of transparent experimental apparatus was designed and fabricated. The velocity of the mixture was investigated by particle image velocimetry ( PIV ) at the air suction flow rate of 9.0 m3/h. The results indicated that the axial velocity distribution of the mixture was a parabola-like curve along the radius and the change of the velocity was larger in the region within 0%～25% of the pipe width (about 15 mm) from the pipe walls, but smaller in other positions. The vortex changed greatly in the region within 0%～16.7% of the pipe width (about 10 mm) from the pipe walls and then decreased gradually to zero at the pipe center.