In recent research, treatment techniques including thermal, ultrasonic, and epoxy-based were adopted in order to enhance the performance of microwave absorber composites. In this paper, the EM properties of absorber composites with micro-sized barium ferrite (BF) fillers are evaluated by a non-contact microwave sensor. First series of composites, with the same compositions, barium ferrite was used without treatment (average particle size 3 μm), in which the barium ferrite content was 0%, 5%, 10%, 15%, and 20%. The barium-ferrite filler that was employed to make the second series is treated with epoxy-based coupling agent LICA38 (1% by weight) instead of being untreated. The microwave sensor characterization indicated that the absorption efficiency and dielectric properties were greatly improved by the epoxy treatment. Enhanced polarization as well as higher microwave energy dissipation were achieved by introducing LICA38, which increased the real part (dielectric constant) and imaginary part (loss factor) of complex permittivity. Treated composites showed an increased absorption by >8%, and the transmission level also decreased by >10% against that of untreated ones. The data indicate that the mechanical, electromagnetic, and dispersion of the fillers are promoted by epoxy surface treatment in microwave absorber composites. The mechanical and magnetic properties of the polymer composite were also improved after treatment with epoxy LICA38. This may help in increasing the BF filler content to improve the magnetic properties of the composite without compromising the mechanical properties of the material.