We report the development of a material testing system forin situelectron microscopy (EM) mechanical testing of nanostructures. The testing system consists of an actuator and a load sensor fabricated by means of surface micromachining. This previously undescribed nanoscale material testing system makes possible continuous observation of the specimen deformation and failure with subnanometer resolution, while simultaneously measuring the applied load electronically with nanonewton resolution. This achievement was made possible by the integration of electromechanical and thermomechanical components based on microelectromechanical system technology. The system capabilities are demonstrated by thein situEM testing of free-standing polysilicon films, metallic nanowires, and carbon nanotubes. In particular, a previously undescribed real-time instrumentedin situtransmission EM observation of carbon nanotubes failure under tensile load is presented here.