A unique domain structure was found in liquid-liquid phase separation induced by radical polymerization of 4-chlorostyrene monomer in the presence of random copolymer poly(dimethylsiloxane-co-diphenylsiloxane). Over a wide range of temperature and initial monomer composition, poly(4-chlorostyrene)-rich droplets with narrow size distribution were generated by phase separation, and these droplets hardly coalesced by collisions. With the growth of droplet size, droplets gradually filled the space and spontaneously arranged themselves into a regular array like crystal. The threshold temperature, below which the array formed, decreased with increasing initial monomer composition, Poly(4-chlorostyrene) had much longer chain length and higher glass-transition temperature than the copolymer component, and noncoalescence of fluid droplets was explained by this asymmetry of viscoelasticity between the segregating components. The regular array was considered to form by a mechanism analogous to the Alder-type fluid-to-solid transition of hard spheres. A possible mechanism of this regular pattern formation was proposed.