[en] We investigated the current–voltage characteristics and the critical current density (J _c) of a YBa₂Cu₃O_7−δ grain boundary (GB) long Josephson junction with respect to magnetic field (H) under different application directions. When H is rotated within the bc-plane of the film, a small peak of J _c is observed when H is parallel to the c-axis, which denotes that there are some residual c-axis dislocations in the GB. When H is rotated within the ac-plane, some vortex-flow steps can be observed in the current–voltage curve at a high inclination angle of H. The vortex-flow step becomes smaller and wider with the decreasing angle between H and the c-axis. These results are a clear demonstration of vortex splitting at the GB plane in YBa₂Cu₃O_7−δ GB long Josephson junctions. (paper)