How can I use a Drupal 6 database in Drupal 7 without upgrading that database? I am new in Drupal. Currently I am working on one project where I am using Drupal 7.12. My client had one project in Drupal 6 of which database he wants to use in the Drupal 7 project. I can not upgrade the Drupal 6 to 7 as it's a completely different instance but he wants to have same userbase and other things in the new project i.e Drupal 7. As per knowledge DB structure of Drupal 6 and Drupal 7 is very different and hence it's not going to be very easy to use the Drupal 6 DB in 7.

Please suggest me any way to use Drupal 6 DB in 7.
Simply put, you cannot unless you are willing to rewrite a lot of Drupal 7 parts.

One thing you can do, is to implement cron jobs to export changes in version-independent format like XML on source site, and import them from XML at target site.

Alternatively, you can use Services module and make one page call the other to perform certain tasks, like user login or article reading.