Ni-P alloy films containing phosphorus particles ͑called "Ni-P alloy composite films"͒ were fabricated by electrodeposition and were subsequently subjected to heat-treatment. Their compositions and microstructures were characterized, and their friction properties were evaluated using a ball-on-plate method. Composite electroplating in the nickel sulfate and chloride bath containing phosphorus acid and micrometer-sized phosphorus particles resulted in the Ni-P alloy coating with enhanced deposit phosphorus content. The phosphorus content of the films increased with increasing phosphorus particle concentration in the composite plating baths, reaching a maximum value of 29.0 atom %. The phosphorus particles were homogeneously distributed in this Ni-29.0 atom % P alloy composite film. Heat-treatment converted the phases of the alloy composite films from an amorphous phase to stable crystalline phases, which are the same as those in the Ni-P binary alloy phase diagram. The friction coefficients of the Ni-P alloy films increased with increasing cycle number, whereas those of the Ni-P alloy composite films remained relatively constant. The alloy composite films had lower friction coefficients than the Ni-P alloy films both before and after heat-treatment. These results indicate that phosphorus particles are beneficial for maintaining a lower and stable friction coefficient during the ball-on-plate reciprocating friction test. Ni-P alloy plating has been extensively used in the industry because it confers good physical and chemical properties, including high hardness and high wear and corrosion resistances. Ni-P alloy films are conventionally fabricated by electroplating and electroless plating.1-8 Most reports discuss alloy films with phosphorus contents less than 25 atom %, which have stable phases of Ni and Ni 3 P according to the Ni-P binary phase diagram. However, the phase diagram indicates that Ni 12 P 5 and other crystalline phases are stable when the phosphorus content of Ni-P alloys exceeds 25 atom %. Although some reports have discussed the microtexture of Ni-P alloy films with phosphorus contents of over 25 atom %, 6-9 the existence of Ni 12 P 5 or other phosphorus-rich nickel-phosphorus compounds has never been reported.Comparison of the microstructures and characteristics of Ni-P alloy films having phosphorus contents greater than 25 atom % with those of alloy films having phosphorus contents under 25 atom % is interesting from both academic and practical viewpoints. We initially fabricated Ni-P alloy films using a conventional electroplating method; however, the maximum phosphorus content that could be achieved by this method was less than 25 atom %.In the present study, to fabricate Ni-P alloy films with phosphorus contents greater than 25 atom %, we used a composite plating method in which phosphorus particles are incorporated into Ni-P alloy films ͑Ni-P alloy composite plating͒. We also analyzed the microstructures of the composite films and evaluated their friction properties both before and after hea...