  The sudden short duration decrease in cosmic ray flux is known as Forbush
decrease which is mainly caused by interplanetary disturbances. Generally
accepted view is that the first step of Forbush decrease is due to shock sheath
and second step is due to Magnetic Cloud (MC) of Interplanetary Coronal Mass
Ejection (ICME). However, there are few questions which needs detail
investigation. (i) Does the complete (or part of) ICME shock or MC contribute
in cosmic ray decrease? (ii) Is there any internal structure associated with
the ICMEs shock/MC which precisely contribute in decrease? To address these
questions we have studied a total of 18 large ($>=8 \%$) Forbush decrease
events and associated ICMEs. Majority of events shows multi-steps Forbush
decrease profile. This leads into re-classification of Forbush decrease events
depending upon the number of steps observed in their respective profile. The
present study clearly indicates that not only broad regions (shock-sheath and
MC) contribute to Forbush decrease but also localized structures within
shock-sheath and MC have very much significant importance in Forbush decrease
profile.
