Ni-P-diamond and Ni-P carbon nanotube (CNT) nanocomposite films have been synthesized and compared in this paper. Because diamond has different shapes and sizes than CNTs, the nanocomposite films exhibit distinct characteristics. Nanoindentation measurements show that the Young's modulus and hardness of the Ni-P-CNT nanocomposite film electroplated in a bath with 0.028 g/L CNTs increase to 665.9±29.5 GPa and 28.9±4.22 GPa, respectively, which is about four times larger than that of pure nickel. On the contrary, the modulus and hardness of the Ni-P-diamond nanocomposite films decrease with the incorporation of the nanodiamond powders due to the formation of voids and cracks, even though both diamond and CNTs have similar Young's modulus and hardness. For the Ni-P-diamond film plated in the bath with 2.0 g/L nanodiamonds, the values are reduced to 110.5±4.3 and 2.3±0.1 GPa. Nevertheless, the electrothermal actuators made of both Ni-P-CNT and Ni-P-diamond composites can provide four times the maximum elongation, larger than that made of pure nickel. With the same displacement, the input power of the nanocomposite actuators is much less than that of the pure Ni one. This study reveals potential applications for nano/microelectromechanical systems fabrication using both Ni-P-diamond and Ni-P-CNT nanocomposites due to their distinctive physical properties and compatibility with contemporary complementary metal oxide semiconductor manufacturing processes.