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MgB2 multifilamentary tapes with 6 twisted filaments were fabricated and it was experimentally proven that both an increase in critical currents and a decrease in ac losses in the tapes due to deformation of the tapes with high aspect ratio can be simultaneously attained. This will improve the performance of MgB2 wires for the new conductors as windings of superconducting coils with both low losses and high stability for fusion reactors, superconducting magnetic energy storage systems, and so on. There are three kinds of prepared samples in which one sample is a round wire of 1.02 mm in diameter and other samples are tapes obtained by the rolling process from the round wire. The aspect ratios of these tapes are 3.3 and 6.6. These wires are composed of MgB2/Nb/CuNi . The 6 filaments are twisted with twist pitches of 50 mm. The measurements of critical currents and coupling losses were carried out at 4.2 K. The observed critical current densities of the tapes are two times larger than that of the round wire under the external magnetic fields of 2 T to 7 T in a direction parallel to the flat faces of the tapes. The observed coupling losses in the tape with aspect ratio of 6.6 were 1/10 smaller than that of the round wires, where the parallel magnetic fields were applied to the flat faces of the tapes.