This report describes a novel polymer nanocomposite that shows high transparency, toughness and dimension stability at the same time. Polymer nanocomposites are attracting industrial interest for a wide range of application. One advantage of polymer nanocomposites is the possibility to obtain visually transparent polymer materials owing to nano-size particles that are smaller than visible wavelengths. Transparent polymer nanocomposites are suitable for light, clear, tough and dimensionally stable applications such as the windows and sunroofs of vehicles or houses and other buildings. However, most of the research on transparent polymer nanocomposites has remained at the stage of showing their potential or future expectations because of the complexity of various nanoparticle-polymer matrix combinations. One of the most difficult challenges for the fabrication of transparent nanocomposites is to keep the dispersion of particles in the polymer uniform at the nano-level until the final composite is obtained. Some successful examples have been reported with polyacrylate-based polymer nanocomposites. For example, silica/PMMA nanocomposites show good transparency even with a high filler content. In fact, most of their transparency depends on refractive index matching. While a nanocomposite may appear transparent to the human eye, TEM observation shows that there are many aggregations inside, preventing the material from ever becoming tough and making it extremely brittle. Some other cases are based on a tough polymer like polycarbonate, but with a very restrained filler content of only several weight percent. Because of such a small amount of filler, dimensional stability cannot be improved. Thus, there have been no reports of highly transparent, tough and dimensionally stable polymer nanocomposites with a filler content of more than 10wt%. We overcame this contradictory challenge and succeeded in dispersing more than 15wt% of filler into polycarbonate. We chose needle-shaped alumina boehmite nanoparticles as the filler, and employed an appropriate amount of organic acid as a surface modifier to aid in uniform particle dispersion in the polymer matrix. Polycarbonate is well-known as a tough polymer, but its main chain easily hydrolyses, either in the presence of an acid or base. The hydrolysis reaction makes the polymer chain shorter, leading rapidly to a brittle composite. We carefully defined the surface modification condition to avoid this hydrolysis reaction. This completely novel composite shows excellent properties without any trade-off, including high transparency (Haze<1%), toughness (Izod>60 J/m) and dimensional stability (CLTE<40 ppm) at the same time.