  Using fully kinetic simulations, we study the x-line orientation of magnetic
reconnection in an asymmetric configuration. A spatially localized perturbation
is employed to induce a single x-line, that has sufficient freedom to choose
its orientation in three-dimensional systems. The effect of ion to electron
mass ratio is investigated, and the x-line appears to bisect the magnetic shear
angle across the current sheet in the large mass ratio limit. The orientation
can generally be deduced by scanning through corresponding 2D simulations to
find the reconnection plane that maximizes the peak reconnection electric
field. The deviation from the bisection angle in the lower mass ratio limit can
be explained by the physics of tearing instability.
