The accurate determination of plutonium (Pu) mass fraction in nuclear materials is essential to nuclear matter accountancy and international safeguard programs. Controlled-Potential Coulometry (CPC) is one of the best available analytical methods to perform such measurements. The implementation of CPC at the Nuclear Matter Metrological Laboratory of the French Atomic Energy Commission is described as well as the evaluation of measurement uncertainty using two approaches: the guide to the expression of uncertainty in measurement, and the Monte Carlo Method. The uncertainty values determined are compared to the international target values published by the International Atomic Energy Agency.