Reaction-induced phase separation of polymer blends was examined by optical microscopy in this work. Four distinct types of ordering processes were observed when a miscible polymer blend was photo-cross-linked in the one-phase region located between the coexistence curve and the glass transition temperatures, These structures, known as modulated phases, emerge from the competitions between phase separation, which acts as a short-range activation process, and the cross-linking reaction, contributing as a long-range inhibition effect. Because of the network formation, elasticity participates as an additional long-range interaction at the late stage of the reactions, making the problem more interesting and complex. These structures and their ordering kinetics observed under various experimental conditions were analysed and classified by the morphological phase diagram constructed from the experimental data. These results are discussed in terms of pattern selection driven by the competitions between photo-cross-linking reactions and phase separation of polymer blends.