Doped LaMnO3 has unusual electromagnetic properties, which makes it possible for this material to be used for absorbing microwave. LaMnO3 systems doped by Sr at site A and Fe or Co, Ni at site B were prepared by sol-gel as an microwave absorption material and their permittivity and permeability spectra were measured by microwave vector network analyzer in the frequency range of 2-18 GHz. A novel phenomenon is discovered that the complex permittivity, complex permeability and electromagnetic loss tangent have suddenly a step change at a certain frequency and the step-change frequency is relevant to content of Sr and Fe or Co,Ni. The samples show mainly dielectric loss when microwave frequency is smaller than the step-change frequency, and mainly magnetic loss when larger than that frequency. It is indicated that anti-ferromagnetic clusters in the material can absorb energy quantum of microwave electromagnetic field to change into ferromagnetic clusters because they can overcome higher energy barrier when the frequency of incident microwave reaches a certain value.