Object Level Restrictions or Permissions

Hi Experts, I have created one Item Type called "Project", which does not have any lifecycle or workflow, the workflow is for Document which is in "Project Document" relationship,  Under which different Identities will work. So, is it possible to restrict User1 to edit other Project Item which is created by User2?  How could I manage this? Both User1 and User2 should work on different Project Item and will not able to access each other Project Item.   Regards, Maddy.