Optical transmission of quartz glass is measured during loading and unloading process，and the stress was kept below their Hugoniot elastic limit. The results show that the quartz glass maintains good transparency for more than 15 μs under loading processes of 18 GPa，then it starts to lose its transparency about 07 μs after unloading，with 30% decrease of transmittance. This phenomenon，which reflects the growth and evolution of failure induced by unloading process in quartz glass，has been explained reasonably by the growth of spherical particles and the scattering model. The results are different from the explanation of liquid-solid phase transition given in literature［J. Chem. Phys. 2004，121 9050］. This paper is significant for studying the transparency of other transparent material.