For the first time, miscibility between poly(styrene-co-acrylonitrile) (SAN) and poly(benzyl methacrylate) (PBzMA) was proved. The miscibility window was found in SAN/PBzMA as the copolymer structure was varied systematically by altering weight percent of the acrylonitrile (AN) unit in the SAN copolymers. The lower bound of AN content in SAN for miscibility of SAN/PBzMA blend is 3.5 wt %, and the upper bound is 24.4 wt % AN in the copolymer. The trend of T-g broadening and LCST (cloud point) behavior for the SAN/PBzMA blends in that these phase-related quantities were all affected by AN content in SAN copolymers. Within the window, the copolymer/polymer mixtures formed a true miscible blend system. The effect of SAN structure (by controlling AN weight percent) on the window miscibility of SAN/PBzMA. blends is interpreted and discussed in detail.