Based on the Wallin model and fracture mechanics theory, a simple power law expression of the effective K R model for steel fibre reinforced cement based composites was presented. To validate this model, a three point bending beam fracture test with different sizes and different steel fibre distributions was carried out. It is shown that the power law model obtained by the K R curves can effectively describe the fracture process of cement based composites for different steel fibre distributions, and the deviation of the peak loads between the theoretical results and the experimental results is small. The presented model provides a new method to study the size effect on fracture parameters of steel fibre reinforced cement based composites.