Abstract: This paper presents an experimental study on fluid mixinginduced by Richtmyer-Meshkov (RM) instability at an interface between waterand air using a rectangular shock tube. The length of mixing zone is foundto obey a linear law h \propto t when the Atwood number approaches to 1under different Mach numbers and initial perturbations. However, the lengthof mixing zone increases when Mach number increases from 1.45 to 1.7, alsoincreases when initial perturbations are random., and the mixing goes moretempestuously.