Abstract: We present an in-situ tensile experiment with a transmission electron microscope on monolithic Zr-based metallic glass at the sub-micron level. It was found that stable and controllable deformation occurred in the sub-micron scale samples which thus exhibited large tensile ductility. This sample-size effect has significant implications for the application of metallic glasses in thin films and micro-devices, as well as for understanding the fundamental mechanical response of amorphous metals.