Materials strength is determined by the chemical bonding and the microstructural features; both can be changed by adding various amounts of constituent atoms. Nanostructure and hardness of titanium aluminum nitride thin films with different compositions prepared by plasma enhanced chemical vapor deposition were investigated in this study. The phase and microstructure of Ti1-xAlxN deposited were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Film hardness and reduced elastic modulus were measured by nanoindentation interfaced with an atomic force microscope. High-resolution transmission electron micrographs show that the grain size of Ti1-xAlxN decreased to less than 10 nm as aluminum contents in the film increased up to a ratio of Al/(Al + Ti) = 0.63. The hardness measurement shows that the microstructure feature was the most important factor in determining the hardness value of the films.