In the recycling process of plastics, grinding is commonly employed as a pre-treatment method to facilitate molding by increasing the specific surface area. A cutter-type disk mill that mainly causes shear forces is expected to be the most efficient for grinding plastics. However, it is necessary to optimize the geometry and operating conditions of the mill. This study aimed to evaluate the grinding performance of different blade geometries in the cutter-type disk mill. To optimize the blade geometry, the discrete element method (DEM) coupled with the computational fluid dynamics (CFD) and a breakage model was employed to simulate plastic particle breakage. The simulations showed that the highest grinding performance was achieved with a grinding blade angle of 45°. Depending on the angle of the grinding blade, the effects of the power acting on the grinding blade and the force acting on the particles varied. According to these results, an appropriate blade geometry in the cutter-type disk mill can be proposed for grinding plastic. This study demonstrated that the DEM-CFD simulations are effective for evaluating the grinding performance and optimizing the blade geometry of the cutter-type disk mill.