A battery pack case of an electric vehicle was developed with a fibrous thermoplastic composite material. Due to cost effectiveness, long-fiber-reinforced thermoplastics by direct process (D-LFT) was adopted. PA6 (Polyamide 6)-based composites were processed using a D-LFT pilot machine at the temperature range between 250° and 290°. Glass and carbon fibers were added in the matrix varying the mixture ratio of the fibers while keeping the weight fraction 40%. The increase of carbon fibers in the mixture increased tensile modulus and strength, however, decreased Izod impact strength. The fatigue life of developed composites was evaluated by fatigue tests in tension, which was over one million cycles at the maximum fatigue loading less than 60% of the composite strength. Associated with fiber orientation, anisotropic mechanical behavior was investigated in terms of flexural properties and mold shrinkage. A shape of strand placement was suggested minimizing the anisotropic behavior for in-plane mechanical properties and mold shrinkage. A design for the lower case of a battery pack case with the composite material was developed. CAE (Computer-aided Engineering) simulations were conducted to optimize the structural performance with respect to impact performance and structural durability. Non-destructive evaluation using X-ray showed no defects in the prototype. It satisfied the requirement of sled and vibration tests. The weight reduction of 31% was achieved.