Order formation in binary mixtures of monodisperse latexes of different particle sizes is studied. A binary latex of a moderate particle concentration is milky white with random distribution of the particles. When the concentration exceeds certain limit, phase separation takes place to deposit an ordered phase or phases with alloy structures. The structures were examined by a light microscope. Structures thus far observed are AlB2, NaZn1 3, CaCu5, MgCu2 and another type which has a composition of AB4 with P63/mmc symmetry. The structures are analysed basing on hard sphere model. It is concluded that (1) structure type is determined by the ratio of effective diameters of the constituent latexes, (2) an alloy structure is a realization of a good state of packing if not of the state of the closest packing and has a higher density than the density of the disordered phase from which it has deposited. This higher density is a cause of the phase transition from disordered to ordered state