A method of molding an interior part for a vehicle includes determining a 3D graphic sample model. A shape of the sample model has concave portions having curvatures determined through 3D modeling and gradients of an inner surface of each concave portion. A molded sample having the same general shape as the sample model is injection molded. Information related to scratch occurrence positions is acquired from the molded sample. Information of curvatures and gradients of the concave portions of the sample model are acquired and collected at positions on the sample model corresponding to the scratch occurrence positions. A limit angle and limit angle are determined by using the collected information and preset curvatures for the concave portion of a design of the interior part. The interior part having the concave portion with a shape based on the determined draft angle and the preset curvatures can then be injection molded.