In an effort to find ways to minimize the undesirable effects of thin metal film stress changes with time on MEMS reliability we have investigated stress relaxation of nanoscale A1 thin films. We have found that the relaxation is strongly dependent not only on temperature but on film thickness as well. Films 33, 107 and 205nm thick prepared by evaporation onto a silicon nitride membrane substrate were studied using membrane resonance to determine film stress. A single thermal cycle to 300°C was used to establish a stress in the aluminum films, after which the time dependence of the stress was measured for the three film thicknesses at 50, 75 and 100°C. The relaxation rate is highest for the highest temperature and the thinnest film. The time dependence is very well represented in all cases by an expression of the form dσ/dt = -A(σ-σ∞)nwhere σ∞ is the limiting stress below which the relaxation mechanism (which we attribute to dislocation motion) is unable to proceed. In all cases the value of n is greater than one so the decays are not exponential. A dislocation locking mechanism is suggested as a possible explanation for the observed thickness dependence.