Rectangular cavity perturbation method was used to measure microwave dielectric loss (MDL) during the solid state reaction synthesis of SrFeCo0.5Oy. In the process of solid state reaction, the dielectric loss is investigated under different temperatures. The phases of the samples synthesized at different temperatures were characterized by XRD. The variation of MDL with temperature illustrates that the phase transformation occurs. The reasons why the pure perovskite phase can be obtained by using microwave processing were also discussed.