Two things to check:

1. Rights. The person creating the custom template-based document
library must be in one of the four built-in Site Groups in the top-level
site. I know it sounds weird but there are ACLs that will prevent the
display of custom templates  even if the user is an Administrator on the
site where the creation will take place. Unless they are at least a
Reader on the top-level site they won't see the template.

2. Site Definition. The custom template  will only be visible on sites
that are based upon the same site definition as the site where the
template was generated. So, for instance, you can't use meeting
workspace templates in a team site or any portal area.

The existing document  library contain so
much documents.

For the time being I copied the document library  manually by using
explorer view .But by doing this the metadata associated with the
document is not copying to new document library

Even I create  the template  for document  library( < 10 mb) the template
is not listing  ???

Do you have rights in the top-level site and is it being created in a
site based on the same site definition?

I am working on Live Server .

I save the document  library as a template
I download the template  to desktop
I upload the template to another site list  template gallery


The template is not listed ???