b-SiAlON with various z-values (z = 0.5~4.0) were produced by hot pressing. The dielectric properties (dielectric constant and tangent loss) of b-SiAlON were characterized by the post-resonator method at room temperature and by the perturbation method from room temperature to 1200 oC at 2.45 GHz, respectively. Effect of z-values and temperatures with b-SiAlON were investigated.