The objective of this research is to optimise the fracture parameters of steel fibre reinforced concretes to obtain a more ductile behaviour than that of plain concrete. The effects of the aspect ratio (L/d) and volume fraction of steel fibre (V-f) on fracture properties of concrete in bending were investigated by measuring the fracture energy (G(F)) and characteristic length (1,h). For optimisation, three-level full factorial experimental design and response surface method were used. The results show that the effects of fibre volume fraction and aspect ratio on fracture energy and characteristic length are very significant. (C) 2003 Elsevier Ltd. All rights reserved.