This article describes a phenomenon that limits the power handling of MEMS resonators. It is observed that above a certain driving level, the resonance amplitude becomes independent of the driving level. In contrast to previous studies of power handling in MEMS resonators it is found that this amplitude saturation cannot be explained by non-linear terms in the spring constant or electrostatic force. Instead we show that the amplitude in our experiments is limited by non-linear terms in the equation of motion which couple the in-plane length-extensional resonance mode to one or more out-of-plane bending modes. We present experimental evidence for the autoparametric excitation of these out-of-plane modes using a vibrometer. The measurements are compared to a modelwhich can be used to predict a power handling limit for MEMS resonators.