Two kinds of rigid polymers, poly (p-phenylene benzobisthiazole), P (PBZT), and poly (4, 4' biphenylene benzobisthiazole), P (BPBZT), and its blend were crystallized isothermally from 0.1 wt% concentrated sulfuric acid solution. The precipitates of pure polymers were unhulled-rice-like discs which were chained to form a straight rod or a rosary. Molecules were oriented normal to the long axis of rod. As structural similarities of P (PBZT) and P (BPBZT), such as a rigid structure, morphology and existence of the axial shift in the crystalline state, the formation of mixed crystals was expected in the precipitate of P (PBZT) / P (BPBZT) blend. Under the condition that the cloud temperatures of both polymers were closed to each other and the crystal growth rates of both polymers were almost the same, the precipitates of blend sample showed a lath-shaped morphology, which differed from either precipitate of pure polymers. Crystalline order of P (BPBZT) component in the precipitates of blend sample was low as same as the precipitate of pure P (BPBZT). The annealing temperature, at which crystallinity of P (BPBZT) component began to increase, was about 100°C higher than that for pure P(BPBZT) precipitate. In addition to crystallites of P (PBZT) and P (BPBZT), a mixed crystal or an ordered state of mixed molecules of both polymers coexisted in the boundary region of both crystallites.