Chalcopyrite compound CuGaTe2 is the focus of much research interest due to its high power factor. However, its high intrinsic lattice thermal conductivity seriously impedes the promotion of its thermoelectric performance. Here, it is shown that through alloying of isoelectronic elements In and Ag in CuGaTe2, a quinary alloy compound system Cu1-xAgxGa0.4In0.6Te2 (0 <= x <= 0.4) with complex nanosized strain domain structure is prepared. Due to strong phonon scattering mainly by this domain structure, thermal conductivity (at 300 K) drops from 6.1 W m(-1) K-1 for the host compound to 1.5 W m(-1) K-1 for the sample with x = 0.4. As a result, the optimized chalcopyrite sample Cu0.7Ag0.3Ga0.4In0.6Te2 presents an outstanding performance, with record-high figure of merit (ZT) reaching 1.64 (at 873 K) and average ZT reaching 0.73 (between approximate to 300 and 873 K), which are approximate to 37 and approximate to 35% larger than the corresponding values for pristine CuGaTe2, respectively, demonstrating that such domain structure arising from isoelectronic multielement alloying in chalcopyrite compound can effectively suppress its thermal conductivity and elevate its thermoelectric performance remarkably.