The dissipative particle dynamics (DPD) simulation method is applied to study the mesoscopic phase formation of cyclic diblock copolymer c-A(m) B-n (m + n = 20). The phase diagram is constructed by simulating at different interaction parameters and composition fractions. The resulted phase diagram is similar to that of the linear diblock copolymer; i.e., the ordered structures such as lamellae, perforated lamellae, hexagonal cylinders, and body-centered-cubic spheres can be identified in the parameter space. Melted structures such as micelle-like, liquid rod, and random network phases have also been found in the phase diagram. The observed (chiN)(ODT) is in agreement with the theoretical prediction, if a finite chain length mapping is applied. Cyclization of a linear block copolymer can induce remarkable changes in the morphology of the organized meso structure. This is attributed to the reduced chain length of the cyclic block copolymer. The existence of the melted structures between totally disordered and the ordered phases emphasizes complex dynamical pathway during microphase separations.