PURPOSE: To obtain a block copolymer containing polybenzazole blocks and blocks of polybenzoxazinone polymers or polyoxadiazole polymers, which does not show any substantial phase separation and exerts a high heat-stability.

CONSTITUTION: A block copolymer containing polybenzazole blocks and blocks of polybenzoxazinone polymers or polyoxadiazole polymers is coagulated from an isotropic dope to form a molded article of a granular composition. The granular composition may be compression molded to form a molded article. This block copolymer may be molded at a high temperature without substantial phase separation, and the polybenzazole polymer provides a support which improves the tensile property of the polymer.