Electron spin resonance (ESR) spectra of the produced BaFe12O19 nanoparticles embedded poly divinylbenzene magnetic film were recorded depending on the ESR power. The ESR experiments were performed at seven different power values from 0.01 to 10 mW. ESR signal amplitude was observed to change exponentially with the applied ESR power. The ESR values of the film were found to be at a microwave frequency of 9225 MHz and a magnetic field of 296 mT. The effective gyromagnetic ratio, geff, of the film was calculated by using the room temperature ESR data. It is concluded that the produced magnetic film has a strong transitions of geff~2.23. The source of the ESR signals are Fe3+ S-state ions belonging to BaFe12O19 nanoparticles.