The Avogadro constant is determined accurately by applying the x-ray crystal density method to 28Si-enriched spheres, which is one of the practical methods for the kilogram redefinition. In this paper, we present the results of mass measurements of 28Si-enriched spheres, performed at NMIJ in 2014 for the determination of the Avogadro constant. The results of reproducibility of the cleaning effects on the mass of the spheres together with the influence of the BIPM Extraordinary Calibrations, are shown. With these achievements, NMIJ plans to participate in the Pilot Study aiming at the kilogram redefinition, where the mass measurements play an important role.