Without a knowledge of the normalized viscosity, a Mach probe may be just a directional probe or a flow meter with large uncertainties. To solve this problem, a Visco-Mach probe has been developed to measure the normalized viscosity ( alpha ) and flow Mach number (Minfinity ) simultaneously. By generating a free pre-sheath and a bounded pre-sheath by a small Mach probe (SMP) within a free pre-sheath generated by a large Mach probe (LMP), one can measure two ratios of current densities: one Rs by the SMP, and the other Rl by the LMP. Then simultaneous deduction of alpha and Minfinity is made by an iteration scheme from Rs and Rl. The applicability of this new probe to the reciprocating probe of JET is discussed