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A new sample assembly for the multianvil high-pressure apparatus has been developed that allows high-strain plastic deformation at high pressures and temperatures with minimal deformation during initial pressurization. The sample assembly consists of a thin slice of specimen sandwiched between pistons with ends cut at 45° with respect to the long axis, surrounded by a Pt tubing and a polycrystalline MgO cylinder. Upon pressurization, uniaxial stress develops due to the anisotropy of mechanical properties. Deformation during initial pressurization that occurred in earlier designs is minimized by introducing soft materials at both ends of the pistons and by the simple-shear deformation geometry (as opposed to uniaxial compression) that allows sliding at the sample-piston interfaces at low pressures. Large plastic strain, up to ~ 100% shear strain, has been achieved in (Mg,Fe)₂SiO₄ at high pressures (up to 15 GPa) and at high temperatures (up to 1900 K). A theoretical analysis has been made to evaluate the relative contribution to specimen strains from the relaxation of elastic strain in a specimen column and from the continuing advancement of the guide-blocks. The observed strain versus time curves suggest that deformation in the present experiments occurred mostly as a relaxation process rather than at a constant strain-rate due to continuous piston movement. A comparison of the creep strength of olivine inferred from the strain relaxation data at 15 GPa and 1900 K with low-pressure data provides an estimate of activation volume for creep as V* ~ 14 x 10^-6 m³/mol. Theoretical analysis shows that constant strain-rate experiments are also feasible by the advancement of the guide-blocks after complete stress relaxation, although the total strain will be much less than that attained in the relaxation process. Possible applications of this technique include for the studies of high pressure rheology and deformation microstructures in high-pressure mantle minerals.