Pt based nanoalloys and trimetallic clusters(e.g.Au-Pd-Pt)have been considered as a promising catalyst for proton exchange membrane fuel cells(PEMFCs)[1-5].In this work,semi-grand-canonical ensemble Monte Carlo(SEMI-GCMC)simulations [6] have been performed to obtain the equilibrium structures of icosahedral Au-Pd-Pt trimetallic clusters with different size and element ratio.