This study evaluated the indentation size effect on the Fe2B/substrate interface using the Berkovich nanoindentation technique. First, the Fe2B layers were obtained at the surface of AISI 1018 borided steels by the powder-pack boriding method. The treatment was conducted at temperatures of 1193, 1243 and 1273K for 4, 6 and 8h at each temperature. The boriding of AISI 1018 steel resulted in the formation of saw-toothed Fe2B surface layers. The formation of a jagged boride coating interface can be attributed to the enhanced growth at the tips of the coating fingers, due to locally high stress fields and lattice distortions. Thus, the mechanical properties achieved at the tips of the boride layer are of great importance in the behavior of borided steel.Applied loads in the range of 10 to 500mN were employed to characterize the hardness in the tips of the Fe2B/substrate interface for the different conditions of the boriding process. The results showed that the measured hardness depended critically on the applied load, which indicated the influence of the indentation size effect (ISE). The load-dependence of the hardness was analyzed with the classical power-law approach and the elastic recovery model. The true hardness in the tips of the Fe2B/substrate interface was obtained and compared with the boriding parameters. Finally, the nanoindentation technique was used to estimate the state of residual stresses in this critical zone of the Fe2B/substrate interface. © 2011 Elsevier B.V.