Nickel slag has a high-content iron and is a secondary utilization resource with great development potential. The coal-based direct reduction is an innovative technology that can be used to utilize the iron resources in nickel slag. The effect of the particle size of nickel slag on the strength and the reduction of nickel slag-coal composite briquettes were investigated. Four samples with particle size of 75~106 μm, 106~150 μm, 150~270 μm, and >270 μm were selected. The drop strength increased 9.4 times and the compressive strength reached 281.1 N when the nickel slag particle size decreased from >270 μm to 75~106 μm. The reduction degree determined by the data from the thermogravimetric experiment indicated that its maximum was 79.545%. The reduction experiments performed at 1200°C for 45 minutes indicated that the nickel slag with particle sizes between 75~106 µm were appropriate for the reduction of the nickel slag-coal composite briquettes.