Rapid alloying in 4d-transition-metal clusters is studied by isoenergetic molecular-dynamics simulations. A many-body potential is proposed by extending Pettifor’s theory of alloys. Numerical results of simulations for two-dimensional clusters by using this potential are reported. It is found that clusters always exhibit surface structural fluctuation at temperatures even much below the melting temperature, which causes every atom in clusters to change its equilibrium position. It is a main driving factor of rapid alloying. Moreover, the presence of enough of a proportion of guest atoms is a necessary condition for the onset of the rapid alloying. The temperature dependence of time spent by alloying (alloying time) is found to obey the Arrhenius law. By extrapolating this result to room temperature, we show that the alloying time is of the order of one second at room temperature, which is consistent with experimental observation.