The phenomenon of stress relaxation in a solid corresponds to a time-dependent decrease in stress under given constraint conditions. An apparatus for stress relaxation compression testing was constructed and utilized to evaluate a series of die cast magnesium alloys. The behavior of the alloys was determined over a 100 hour test period. In general, the percentage of stress remaining after this time decreased with increased initial stress and with increased test temperature. Of the materials tested, magnesium alloy AE21 showed the best relaxation resistance relative to 383 aluminum alloy.