Even as gigahertz (GHz) acoustic streaming has developed into a multi-functional platform technology for biochemical applications, including ultrafastmicrofluidicmixing, microparticle operations, and cellar or vesicle surgery, its theoretical principles have yet to be established. This is because fewstudies have been conducted on the use of such high frequency acoustics inmicroscale fluids. Another difficulty is the lack of velocimetrymethods for microscale and nanoscale fluidic streaming. In thiswork, we focus on the basic aspects of GHz acoustic streaming,including its micro-vortex generation principles, theoretical model, and experimental characterization technologies.We present details of a weak-coupled finite simulation that represents our current understanding of the GHz-acoustic-streaming phenomenon. Both our simulation and experimental results show that the GHzacoustic-induced interfacial body force plays a determinative role in vortex generation. We carefully studied changes in the formation of GHz acoustic streaming at different acoustic powers and flow rates. In particular,we developed a microfluidic-particle-image velocimetry method that enables the quantification of streaming at the microscale and even nanoscale. This work provides a full map of GHz acoustofluidics and highlights the way to further theoretical study of this topic.