Dear sir,

                 Though  i m following the below procedure getting the same error " You cannot create a new Master-Detail relationship on an existing custom object if records already exist. You must first create a Lookup relationship, populate the lookup field with data in all records, and then change the relationship type to Master-Detail. "

goto setup--> App setup-->create-->object-->objectname-->field(wat u wnt change to convert the existing object from lookup field to master-field reationshipfieldname)edit--->select(masterdetail--->next---->next--->next-->save) this is the process of look up relation to master detial if one object  2 times only master details  existing

Regards,

Jashmanth sarkar