I cannot delete my custom object though I have deleted all look ups to all other object yet it says it has relationship with other objects.
A similar question was asked a couple of days ago.
You might want to try the solution offered by crmprogdev where he says:
You might want to try one of the numerous free Apps from the App Exchange to search the metadata like Octopus or Schema Surfer to locate the template (or anything else) that's causing the issue you're having.
Try using one of these that might find whatever is linked that is causing this problem.
Alternatively, make sure there's nothing like Formula Fields on other objects referencing the object you want to delete, Master-Details relationships, anything like that. Lookups might not be the only cause of the problem.