Three-dimensional measurement of microstructures requires a 3-D microprobe with small probing ball as well as appropriate nanopositioning and nanomeasuring machines. An electric arc melting method is presented in this paper, which could fabricate the monolithic micro-spherical tip on the tungsten wire. Based on the principles of electrode discharging energy absorption and the surface tension phenomenon, a micro sphere is formed at the needle-shape tip of the tungsten wire. The model of dynamic arc discharging is established to analyze the process of sphere forming. Experiment results reveal that a spherical tip about 60 μm in diameter having less than 1 μm roundness error could be achieved on a 100 μm diameter tungsten wire with the selection of proper process parameters, such as the discharging voltage, discharging time and discharging gap. Quality of the probe is mainly determined by the electro-discharge conditions which affect the solidification force and thermal conductivity. The monolithic microprobe can be used in the micro coordinate measurement machines (Micro-CMMs) to allow the measurement of micro grooves possible.