This report describes the application of magnetic compound fluid (MCF) rubber as a material for microwave heating. MCF rubber is one of several new composite materials utilizing MCF as a newly developed magnetic responsive fluid developed by Shimada. The temperature of water in a PP container enveloped by MCF rubber in a microwave oven was measured at various mass concentrations of magnetic particles in the MCF rubber. The measurements were compared to those of MCF rubber in a microwave oven. The results showed an optimum mass concentration in relationship to the increase or decrease of the water temperature. At less than about 50 wt% and more than about 90 wt% of the magnetic particles in the MCF rubber, changes in the water temperature were small but those of the MCF rubber were large. However, in the range of about 60-80 wt%, changes in the water temperature were large but those of the MCF rubber were small. To explain the experimental data, we derived a simple microwave heating theory and measured experimental data for the complex permittivity and permeability of the MCF rubber. This could partly explain the experimental results qualitatively. We also found in the present study that iron particles are better for the MCF rubber in microwave heating than copper or nickel particles. In conclusion, the MCF rubber is a useful material for microwave heating in a microwave oven.