The idea of repulsion in random copolymers was applied to the miscibility modification between polystyrene (PS) and polyarylate (PAr) segments of PS-PAr block copolymer (PAr-PS-PAr). Acrylonitrile (AN), which has a large positive interaction parameter against styrene, was used as a miscibility modifier toward PAr segments. AN was introduced into the carboxyl terminated telechelic-PS at AN wt % ranging from 12 to 37 wt %. Based on these telechelic acrylonitrile-styrene random copolymers (SAN(x)'s where x represents AN wt %), SAN(x) and PAr block copolymers (PAr-SAN(x)-PAr's) were synthesized. The miscibility of SAN(x) and PAr segments was estimated from the results of DSC with Fox's equation and spin-spin relaxation time measured by pulsed NMR. These results evidenced that the miscibility between PS and PAr segments can be modified by introducing AN into PS segments. The estimated volume fraction of the interfacial layer between SAN(x) and PAr segments was increased as x was increased toward 24 wt %, around which the predicted miscibility reaches a maximum. Above that AN wt %, it began to decrease. The flexural strength increased as the miscibility between SAN(x) and PAr segments increased. In particular, when x was between 20 and 30 wt %, PAr-SAN(x)-PAr exhibited three times larger flexural strength than PAr-PS-PAr. The fracture behavior changed from brittle to ductile, even though the telechelic SAN(x) by themselves exhibited almost the same fracture strength as the telechelic PS. The results of dynamic mechanical measurements and the percolation model suggested that around these AN wt % the continuum matrices in PAr-SAN(x)-PAr changed from SAN(x) phase to a cocontinuous phase of SAN(x) and PAr. From these results, PArSAN(x)-PAr was explained to perform such a high flexural strength by this phase change in the continuum matrices.