Abstract: In order to determine uranium, plutonium, boron and other elements with isotope dilution method, the spikes of very little amount should be weighing accurately and precisely to 0.cling level. In this paper, the methods for correction of air buoyancy, for minifying the influences of the temperature and for correction volatilization effect of the samples are discussed. Expressing methods of the measurement uncertainties in every measurement and calculation result are also given.