A two-dimensional self-consistent-field lattice model incorporating the rotational isomeric state scheme to account for chain rigidity was developed to study the phase diagram of rod-coil diblock copolymer melts. Several morphologies such as lamella, zigzag lamella, elliptical cross-sectional cylinders, and hexagonally packed cylinders were observed for rod-coil diblock copolymers. A free energy analysis suggests that the zigzag and elliptical cross-sectional cylindrical structures are metastable. The effect of decreasing the flexibility of the coil segment of a rod-coil diblock copolymer was determined. Our calculation showed that as the flexible block of the copolymer becomes more rigid, part of the cylindrical domain in the phase diagram is replaced by lamella structures.