Grain boundary electrical conductivity of ferrite materials has been characterized using scanning microwave microscope. Structural, electrical, and magnetic properties of Fe3O4 spin-sprayed thin films onto glass substrates for different length of growth times were investigated using a scanning microwave microscope, an atomic force microscope, a four-point probe measurement, and a made in house transmission line based magnetic permeameter. The real part of the magnetic permeability shows almost constant between 10 and 300MHz. As the Fe3O4 film thickness increases, the grain size becomes larger, leading to a higher DC conductivity. However, the loss in the Fe3O4 films at high frequency does not increase correspondingly. By measuring the reflection coefficient s11 from the scanning microwave microscope, it turns out that the grain boundaries of the Fe3O4 films exhibit higher electric conductivity than the grains, which contributes loss at radio frequencies. This result will provide guidance for further improvement of low loss ferrite materials for high frequency applications. © 2014 AIP Publishing LLC.