While micromachined accelerometers are widely available and used in various applications, some biomedical applications require extremely small dimensions (3) ever published. We achieve miniaturization by using a film encapsulation technique with a thick epitaxial polysilicon layer. This packaging technique enables the dimensions of the die to be only tens of microns larger than the micromechanical structure. We have fabricated accelerometers as small as 0.034mm3 (387μm×387 μm×230μm) with noise floor of 0.25mg/√Hz. These ultra-miniature motion sensors have potential opening up new frontiers in biomedical science and engineering.