Abstract: The evolutions of the Richtmyer-Meshkov instability of two kinds of membrane-less heavy gas cylinder (SF6 and argon),under reshock condition with a specific reflected dis-tance,are experimentally studied in a horizontal square shock tube.Based on the jet technique, SF6 and argon gas cylinders are generated respectively to study the effect of Atwood number on the development of the Richtmyer-Meshkov instability.For the visualization of the flow,the tested gases are mixed with glycol droplets which are generated by the fog generator.Illuminated by a continuous laser sheet with a width of 80mm and a thickness of 1 mm,the interface mor-phologies after incident shock and reshock impact are captured in a single test run with the help of the high speed camera.The results show that different evolving rates and interface morphologies before and after reshock are observed for SF6 and argon gas cylinders due to the different Atwood numbers.For the same reflected end wall,the visible two reversed vortex rings are generated for the SF6 gas cylinder after the incident shock passage,which are not apparently observed for the argon gas cylinder because of less vorticity deposition on the interface.Moreover,after the re-shock impact,secondary vortex rings which have reversed rotating directions to the original vor-tex rings are quickly generated in the SF6 gas cylinder and dominate the flow field at the later stage,becoming the primary feature of the flow.However,in the argon gas cylinder,the second-ary vortex rings,though generated with opposite directions of rotation to the original ones,are not fully developed all the time due to the smaller Atwood number that results in less production of reversed baroclinic vorticity on the argon interface.The present results illustrate the pronounced influence of the Atwood number on the development of the Richtmyer-Meshkov instability.