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The elastic wave velocities of polycrystalline MjPy garnet along the majorite-pyrope system have been measured at pressures up to 21 GPa and temperatures up to 2,000 K using ultrasonic interferometry in conjunction with in situ X-ray diffraction techniques in a Kawai-type multi-anvil apparatus. The elastic moduli of MjPy garnet and their pressure and temperature derivatives are determined by a two-dimensional linear fitting of the present experimental data, yielding: K = 161.5 (7) GPa , ∂K/ ∂P = 4.42 (4) , ∂K/ ∂T = −0.0154 (2) GPa/K , G = 86.2 (2) GPa , ∂G/ ∂P = 1.28 (1) , ∂G/ ∂T = −0.0096 (5) GPa/K. The present results together with those of the studies on the majorite-pyrope solid solutions suggest the pressure and temperature derivatives of elastic moduli are insensitive to the majorite content in the majorite-pyrope system. The velocity gradients of the majoritic garnets in the majorite-pyrope system are 3 ~ 6 times lower than those required to account for the high seismic velocity gradients observed in the mantle transition zone. [ABSTRACT FROM AUTHOR]