In this study, randomly organized and uniaxially aligned polyacrylonitrile (PAN) nanofibers with an average diameter of similar to 550 nm were prepared by electrospinning. They were embedded into poly(methylmethacrylate) (PMMA) to fabricate reinforced composite films by solution impregnation technique. Though the PAN nanofibrous mats were white in color, and fiber contents in composites were as much as 16 wt%, and the refractive index (RI) was different between PAN (RI = 1.5187) and PMMA (RI = 1.489), the composite films showed excellent optical transparency with as high as 90% light transmittance in the visible light range, regardless of the fiber orientation. The mechanical properties of composite films steadily improved with nanofiber contents, and were dependent on the fiber orientation. The tensile strength and Young's modulus of the composite film containing 16% aligned fibers were respective 40% and 30% more than that with the corresponding amount of randomly organized fibers. The embedding of PAN nanofibers displayed no effect on the glass transition temperature of PMMA matrix, but increased the stiffness of the composite films in the glassy and glass-rubber transition state, as demonstrated by the dynamic mechanical analysis. This optically transparent PMMA composite with improved mechanical strength may be potentially used as the windshield for the highly moving objects.