Abstract: The estimator of population standard deviationcan be represented by sample standard deviation s, s/c_4, \olR/d_2 and \wh\sigma_\textTOTAL based on MVA, espectively. This paper presents the estimation of C_p and C_pk as well as the confidence interval of C_p using different estimation methods of population standard deviation. Further, each estimation method is analyzed in an example.