The screening effect on 3D structure of charged dust particles confined electrostatically in a plasma is studied using a particle simulation. We find that a few hundred particles form multi-shell structure in the presence of screened Coulomb potential, and that the mean radius of the shell is given by R-s infinity (1 +D-infinity/Lambda(D)) exp(-D-infinity/Lambda(D)), where R-s infinity is the radius of the sth shell observed in the bare Coulomb potential, Lambda(D) is the screening length, and D. is the interparticle distance on the sth shell for Lambda(D)->infinity.(c) 2005 Elsevier B.V. All rights reserved.