  We have studied anisotropic magnetoresistance (MR) and magnetization with
rotating magnetic field (B) within $CuO_2$ plane in lightly doped AF
$Nd_{2-x}Ce_xCuO_{4}$. \emph{A giant anisotropy} in MR is observed at low
temperature below 5 K. The c-axis resistivity can be tuned about one order of
magnitude just by changing B direction within $CuO_2$ plane and a scaling
behavior between out-of-plane and in-plane MR is found. A "Spin valve" effect
is proposed to understand the giant anisotropy of out-of-plane MR and the
evolution of scaling parameters with the external field. It is found that the
field-induced spin-flop transition of Nd$^{3+}$ layer under high magnetic field
is the key to understand the giant anisotropy. These results suggest that a
novel entanglement between charge and spin dominates the underlying physics.
