Process capability indices (PCIs) have been extensively used to determine whether the process products meet the specifications limits, and they can provide decisions for quality assurance problems and guide a principal for improving process quality. Among various indices, Cpm is one popularly used index because it provides a measure on the process loss, but the estimator of it is biased. Thus, this thesis, proposes a correction factor to obtain the approximately unbiased estimator. The curve fitting by the computer program Matlab is adapted to obtain the correction factor. Since the process parameters μ and σ are unknown in the correction factor, bootstrap method can be used to estimate them.