According to the experimental facts that the structure and the dielectric property of electro-rheology (ER) fluid change under the external electric field, a model of microwave transmittance in ER fluid was established, and the formula of transmittance was given also. Theoretical simulation shows that the transmittance of microwave increases with increasing electric field, if the dielectric constant of ER fluid is greater than that of its container. On the contrary, it decreases with the electric field. Our experiment results revealed that the microwave attenuation in ER fluid could be adjusted by changing the external electric field. We think the change of the structure and dielectric property of ER fluid are the main cause of the fact that the transmittance could be adjusted under the external electric field.