Abstract: Numerical investigations on the near wake of a circular disk in a shear flow at low Reynolds numbers were performed using DNS, with focus on the effect of shear rate on the wake structures. The Reynolds numbers considered were 140, 160 and 300, and the non-dimensional shear rate k of inlet flow was varied from 0 to 01. At Re=140 and 160, the wake changes to the unsteady state with planar-symmetry, indicating that the critical Re of this mode decreases in the uniform shear flow. The head of the hairpin vortex loop is always much larger on the high-velocity side within the shear flow, and at Re=160 the planar symmetry gradually moves to the xoy plane with the increase of the shear rate k. At Re=300, no obvious changes of the wake structures are observed, and the wake is still in the weakly-chaotic state. It is also found that, for low values of k (k<01), the non-dimensional vortex shedding frequency of the shear flow is nearly the same as that of uniform flow.