The mechanical behavior, including the tensile and torsional properties, of bulky wool singles yarn, which has an initial fiber packing density that is non-uniform along yarn radial direction, is modeled in this paper. The theoretical analysis is based on discrete-fiber- modeling principles by which the yarn is considered as an assembly of a large number of discrete fibers. The movement of fibers during deformation and their final positions after deformation are determined by their need to minimize tensile strain, which is the so-called the shortest-path hypothesis. Energy method is employed to calculate the applied force. Taking the nonlinear behavior of fiber at large strain into account, the contributions to yarn external force due to fiber tension, fiber bending and fiber torsion can all be derived. Comparing with the experimental data, our theoretical model gives a reasonably accurate prediction of yarn behavior at small deformation.