We propose a technique for evaluating the compatibilization effect of maleic anhydride-grafted polypropylene (MAPP) introduced into glass fiber reinforced polypropylene by probing chemical structures using Fourier transform infrared (FTIR) microscopy. The dispersion state of MAPP in the composites was investigated by FTIR imaging constructed with integrated spectral intensity arising from carbonyl groups. MAPP was predominantly distributed around the glass fibers, due to the formation of interactions between the carbonyl groups in MAPP and the surface silanol groups on the fibers. The effects of interfacial interactions on matrix-fiber adhesion of the fiber composite were confirmed by observation of a fracture surface, using FTIR imaging, after tensile testing. The presence of polymer adhered to the fiber was detected. This revealed cohesive failure to occur in the matrix portion, attributable to improved interfacial adhesion. The addition of MAPP is therefore essential to improve matrix-filler adhesion, which contributes to enhanced strength and ductility by efficient stress transfer and prevention of fracturing at the matrix-fiber interface. This characterization technique can be utilized as a generally applicable tool to reveal the mechanisms of compatibilization of polymer composites.