Based on the analysis of the fractile geometry theory, we have measured many machined workpieces with rough surface by different sampling length and data acquisition number. The experiment results show that the conventional roughness parameter Ra dependents on the sampling length. However, the fractile dimension D is an instrinsic parameter for characterizing rough surfaces, whichis indpendent of sampling length. Although it varies with the data acquisition number,the D tends to a constant value as the data acquisition number is large enough.