Electron Spin Resonance (ESR) spectroscopy was applied for monitoring the orientation and distribution of filler particles in polymer composites by measuring the magnetic anisotropy of naturally occurring Mn(II) centers in CaCO3 and in talc. The amplitude ratio of characteristic ESR bands gives the order parameter. The orientation of the particles changes as a function of composition, depends on processing technology, the type of molding (injection vs compression molding) and has a specific spatial distribution in the cross-section of the injection molded specimen. Correlation is found between average orientation of anisotropic particles and the mechanical properties of various composites.