The porous NiTi alloys were prepared by microwave sintering with different Ni/Ti powder particle sizes, and the effect of particle size on the microstructure and mechanical properties of the porous NiTi alloys was systematically investigated. The results show that the undesired Ti2Ni and Ni3Ti phases in the porous NiTi alloys decrease and the Ni phase disappears with decreasing the particle sizes. At the same time, the pore shapes of the porous NiTi alloys change from the irregular shape with sharp angles to the suborbicular shape. Moreover, both of the porosities and pore sizes of the porous NiTi alloys increase with increasing the particle sizes, while all of the Rockwell hardness, compressive strength and bending strength decrease. Therefore, the decrease of particle sizes can be beneficial to obtain the microwave sintered porous NiTi alloys with desired microstructure (pure phase composition and uniform pore structure) and high mechanical properties.