Phase separation of anthracene-labeled polystyrene/poly(vinyl methyl ether) (PSA/PVME) blends was induced by photo-cross-linking the PSA chains with linearly polarized light in the one-phase region of the blend. The cross-linking reaction was performed by taking advantage of the polarization-selective photodimerization of anthracene labeled on the PSA chains. Under this particular irradiation condition, the spatial symmetry of concentration fluctuations was broken, resulting in anisotropic phase separation directed by the exciting polarization. By comparison of these experimental data with those obtained by using unpolarized light under the same conditions, it was found that the morphology induced by linearly polarized light reflects the interference between instabilities propagating with different rates along different directions in the reacting blends.