Do this in terminal, if you use Drush 


 drush delete-all [content-type-machine-name]


 Examples:
 drush delete-all article Delect all article nodes.
 drush delete-all all Delete nodes of all types.
 drush delete-all --reset Delete nodes of all types, and reset node, revision and comment counters.
 drush delete-all users Delete users.
 
 Options:
 --reset Reset counter for node, revision and comment tables.
 --roles pick roles