By using the Collins' model, Kawamura had studied the phase transition of a two dimensional system. In this work, taking into account the molecular fraction and the presence of the 4-atom, we calculated the Gibbs free energy and studied, the influence of the percent of 4-atom on the parameters of the triple point and the critical point. We obtain a phase diagram showing three phases with a triple point and a critical point. The relation between the extent of each phase and the temperature is calculated along each transition line. The obtained results are quite analogous to the behavior of the three-dimensional substances.