Ni-B alloy films containing amorphous boron particles (referred to as "Ni-B alloy composite films") were fabricated by electrodeposition and were subsequently subjected to heat-treatment. Their compositions and microstructures were characterized, and their hardness was evaluated. The content of boron particles in the alloy composite films increased with boron particle concentration in the plating baths. In addition, the total boron content in the films increased with decreasing current density, reaching a maximum value of 34.3 atom %. The boron particles were homogeneously distributed in these alloy composite films and exhibited no cohesion. Heat-treatment of the alloy composite films consisting of a Ni-B alloy matrix and the boron particles led to a phase conversion from an inhomogeneous amorphous phase to stable homogeneous crystalline phases, which were similar to those in the Ni-B binary alloy phase diagram. The hardness of the Ni-B alloy 34.3 atom % B composite film was higher than that of a Ni-B alloy film both before and after heat-treatment.