We fabricated and characterized three kinds of nanocrystalline (NC) Ni-B electrodeposits with low boron content (i.e., 0.05, 0.12, and 0.19 wt.%) for micro-electromechanical system (MEMS) applications. The fabrication was performed by electrodeposition in a nickel sulfamate solution, adjusting the concentration of the boron source, dimethyleamine borane (DMAB). The electrodeposits exhibited high thermal stability, being able to maintain their grain size at 573 K, whereas pure NC Ni started recrystallizing at 473 K. This can be explained by the formation of Ni x B precipitators in the nickel matrix, which then migrate to the grain boundary region and effectively impede grain growth at high temperatures. Furthermore, the mechanical properties of NC Ni-B electrodeposits were significantly enhanced and they exhibited excellent mechanical performance. The hardness and elastic modulus of Ni-0.19B with 573 K heat treatment are 251.4 and 12.6 GPa, respectively, which are much higher than those of pure NC Ni. These outstanding properties show the potential applicability of NC Ni-B electrodeposits as microgears, micocantilevers, microactuators, and MEMS probe cards, which require high mechanical performance and thermal stability.