In this talk, researches on the mechanical behavior of micronanoscaled metallic glasses (MGs) will be reviewed, with an emphasis on those achieved through in situ transmission electron microscope. It was found that along with the reducing sample size, the deformation and fracture of metallic glasses will change from brittle to ductile under both compression and tensile conditions. In addition, the tenet of “smaller is stronger” observed in crystalline materials also holds true for metallic glasses in certain sample size range below which the “smaller is softer” was observed. With appropriate sample design, the theoretical high elastic strain limit can be achieved experimentally through uniaxial tensile tests. When metallic glasses were subjected to cyclic straining, nanoscale extrusion-like roughening can be observed in the highly deformed ahead of a pre-designed starter notch, followed by crack nucleation and formation of nanocrystalline grains. Finally, some unsolved issues will be discussed.