Morphology and phase separation kinetics of anthracene-labeled polystyrene/poly (vinyl methyl ether) (PSA/PVME) blends were investigated by means of phase-contrast optical microscopy combined with digital image analysis. A PSA/PVME mixture with an off-critical composition (40/60) was irradiated with ultraviolet light in the one-phase region to induce the photodimerization of anthracene moieties labeled on the PSA chain. It was found that the irradiated blend exhibits co-continuous morphology with multiple characteristic length scales. Furthermore, these modulated structures were frozen at an intermediate stage of phase separation due to the soft modes suppression of the reaction. These experimental results are in good agreement with the prediction by linear stabilty analysis of reaction-diffusion equations proposed for phase separating systems. The discretization of concentration fluctuations induced by photo-cross-linking reaction reveals promising methods using chemical reactions for morphology control of polymer blends.