This paper proposes a sketch-based interface for modeling muscle fiber orientation of a 3D virtual heart model. Our current target is electrophysiological simulation of heart and fiber orientation is one of the key elements to obtain faithful simulation results. The interface and algorithm are designed based on the observation that fiber orientation is always parallel to the heart surface. The user specifies the fiber orientation on the surface by drawing a freeform stroke on the object surface. The system first builds a vector field on the surface by applying Laplacian smoothing to the mesh vertices and then builds volumetric vector field by applying Laplacian smoothing to the voxels. The usefulness of the proposed method is demonstrated through a user study with a doctor in that area.