The effects of accuracy of measured fractal dimension D and roughness exponent H are investigated in this paperwith a view to examine the reliability of D and H as materials dimensionless parameters of fracture surfaces.Dand H are different from general physical quantity,because they are dimensionless quantities and often appear asexponents in a theoretical function or formula.In many cases,the error of the physical quantity related to D or Hmay far exceed 10％,if D or H has error around 10％.The required accuracy of fractal dimension and roughnessexponent should be higher,but it depends on the specific material,the associated physical quantity and the scale ofmeasurement The effects of accuracy of measured fractal dimension D and roughness exponent H are investigated in this paper with a view to examine the reliability of D and H as materials dimensionless parameters of fracture surfaces. Dand H are different from general physical quantity, because they are dimensionless quantities and often appear asexponents in a theoretical function or formula. In many cases, the error of the physical quantity related to D or Hmay far exceed 10%, if D or H has error around 10%. The required accuracy of fractal dimension and roughnessexponent should be higher, but it depends on the specific material, the associated physical quantity and the scale ofmeasurement