In case of grinding by a single abrasive grain for brittle materials, the dynamometer must sense extremely small amount of grinding force. It also must respond to abrupt changes of force at high frequency caused by cracking of ground surface. In this paper, a piezo-electric dynamometer just for grinding by a single abrasive grain was newly developed. From the matching of the position of observed cracks with the output peaks of grinding forces, it was confirmed that the dynamometer satisfied the requirements mentioned above. A new calculation system was proposed for measuring impulsive grinding force from output voltage out of piezo-electric devices. It was applied for a single grain grinding on MgO single crystal. A method of correction for interference between horizontal and vertical components of grining force was also proposed.