Abstract: With the high-resolution millimeter-VLBI observations, we have determined an intrinsic size of the radio-emitting region of the galactic center compact nonthermal radio source Sagittarius A* (Sgr A*) of only 1 AU in diameter at 3.5mm, strongly supporting its supermassive black hole nature. A full description of this work is given, along with a brief historical overview of our understanding of black hole physics over the past two centuries since the concept of black holes was first pursued in 1783. Future sub-millimeter VLBI observation is promising for revealing the fine structure of the shadow of the galactic center black hole.