Abstract: A new gravimetric method for determining Cu and Sn in the core-shell Cu-Sn bimetallic nanoparticle was proposed. The Cu nanoparticle acts against its routine properties and can be dissolved by hydrochloric acid. By adding the reductive agent, sodium hypophosphite, into hydrochloric acid, Sn of the particles is dissolved completely, and Cu entirely leaves in the remains. So dip Cu-Sn bimetallic particles in this acid, stir completely, then dry and weigh the remains. In this way, Cu and Sn in the bimetallic particles can be determined. For one sample, deviation for three parallel experiments is less than 0.2%, difference between the result of this method and that of compleximetry is no more than 0.1%.