The piston prover was developed and the flow measurement uncertainty of this piston prover was evaluated according to ISO/IEC 17025. The laser interferometer, instead of the optical sensors used in the typical provers, was employed in this prover to measure accurately the testing time and the moved distance of the piston. Uncertainty was calculated with evaluation of various uncertainty factors affecting flow measurement. The expanded uncertainty (U) of the piston prover was $1.3{ imes}10^{-3}$ (at the confidence level of $95\%$). This evaluation example will be useful in the flow measurement uncertainty determination of other gas flow measurement system.