Abstract: The SMM observation of the 18 August 1980 coronal mass ejection is analyzed thoroughly and quantitatively,using the function fitting and digital filtering tech niques.It is shown that the formation of an asymmetrical high density cloud is something related to the deviation in the location of eruptive prominence with respect to the asymmetrical axis of the streamer.The evolution of the asymmetrical structure and its influence on the development of the event are discussed.The dark cavity is proved to be an expanding structure with strong magnetic field. From the radial brightness profiles,the first observational evidence for the front steepening of a non-linear coronal wave during its evolution is found with the time and height for shock formation estimated.The results of the present study are in favor of the event driven by an expanding strong magnetic field structure,rather than by a thermal impulse.