The formation and stability of the ordered structure LS5 composed of large and small monodispersed spherical particles L and S, respectively, in aqueous solutions are investigated, the Brownian motion of particles with the interparticle potential being simulated by the Monte Carlo method. It is found that the formation of the ordered structure LS5 is successfully simulated and the conditions within which the ordered structure is stable are clarified. The existence of another ordered structure of LS5 beside the structure which has been observed experimentally is suggested by the simulation.