Quite recently, the high thermoelectric performance has been reported in the MgAgSb-based half Heusler alloys. The ZT values close to 1 at room temperature with a maximum of ZT = 1.4 at 475K make them a possible candidate as room-temperature thermoelectric generator. But their crystallographic structure and dynamics are rarely known, except a structural study on impurity sample above the room temperature up to now. By virtue of our single phase samples, we hope that those MARI measurements will map out the PDOS softening and broadening with increasing temperature to justify the effect of anharmonicity and GEM measurements will to get isotropic mean-square displacement and even anisotropic thermal factors, as a function of temperature, to judge whether there exists the static and/or dynamic displacement, thus also test extensively the speculation of atomic relaxation in nanostructure.