Based on the results of previous single complexing agents, we prepared mixed complexing agents for electroless NiB plating based on sodium citrate, and studied their characteristics. Complexing agents were mixed as follows: sodium citrate-sodium malonate, sodium citrate-glycine, sodium citrate-diethylenetriamine. The deposition rate from the mixed complexing agent bath was influenced by pH and complexing agent concentration, and mainly corresponded to mixed behavior except for sodium citrate-diethylenetriamine. The boron content of deposited films gradually increased with increasing sodium citrate concentration for the mixed complexing agent. The carbon content was highest as 0.72wt% at a ratio of 1:1 for sodium citrate-diethylenetriamine. The microstructure and crystallization by heat-treatment of NiB films mainly depended on boron content for the mixed system. Codeposited carbon in NiB films may also affect the microstructure and crystallization behavior.