Poly(methyl)methacrylate (PMMA) composites with various contents of titanium dioxide (TiO₂) were prepared. The effects of fiber content and modification on the mechanical properties and thermal behavior of PMMA composite film were studied. The use of organosolv lignin and acrylic acid together as a binary modifying agent for TiO₂ filler increased both the strength and the ultimate strain of treated PMMA composites as compared to the untreated ones. The higher temperature dramatically increased the weight loss of all three sample categories. The organosolv lignin and acrylic acid mixture was more effective than a single modifying agent in reducing the thermal decomposition of PMMA/TiO₂ composites.