Abstract: High purity copper electrode was used to braze 10 steel with brazing gaps of 0.05–1.00 mm. The effects of brazing gap on the formability, interfacial microstructure and mechanical properties of brazed joints were studied. The results show that when the brazing gap was less than 0.10 mm, the brazing metal could not be fully absorbed into the gap, resulting in poor local infiltration. When the brazing gap was 0.10–0.30 mm, the interface of brazed joints was well infiltrated and the brazing seam was full. When the brazing gap was too large (greater than 0.50 mm), the brazing metal collapsed after melting, and the joint filling was not enough to fill brazing gaps to form a continuous brazed joint. When the brazing gap was 0.15–0.30 mm, the tensile strength of the joints was 324–345 MPa. When the brazing gap was smaller then 0.15 mm or larger than 0.30 mm, the tensile strength was relatively low. During brazing, interdiffusion of Fe and Cu between the brazed joint and the base metal occured, and dispersed Fe particles were precipitated in the brazing sean near interface while the grain boundary in the base metal was thickened. No tearing at the interface was observed in the specimens after tension fracture. Considering the brazing formability and joint strength, the brazing gap when brazing 10 steel with high purity copper electrode was suitable to be controlled in the range of 0.15–0.3 mm.