  Low-temperature specific heat is measured on the overdoped
Ba(Fe_{1-x}Co_x)_2As_2 (x = 0.13) single crystal under magnetic fields along
three different directions. A clear anisotropy is observed on the field
dependent electronic specific heat coefficient {\gamma}(H). The value of
{\gamma}(H) is obviously larger with magnetic field along [001] (c-axis) than
that within the ab-plane of the crystal lattice, which cannot be attributed to
the effect by anisotropy of the upper critical field. Meanwhile, the data show
a rather small difference when the direction of the field is rotated from [100]
to [110] direction within the ab-plane. Our results suggest that a considerable
part of the line nodes is not excited to contribute to the quasiparticle
density of states by the field when the field is within the ab-plane. The
constraints on the topology of the gap nodes are discussed based on our
observations.
