In salesforce, can we convert Look up field into Master detail? If Yes Then How?

Hi suraj

You can convert a lookup relationship to a master-detail relationship, but only if the lookup field in all records contains a value.

And also We can convert the lookup relationship to master detail relationship only if all the existing record has valid lookup field values.

For better undertstanding refer to this link:http://opfocus.com/converting-lookups-to-master-detail-relationships/

Hope this may help you:

Hi

To convert lookup to the master-detail relationship you need to give parent record on all child records. This is because a master-detail relationship does not allow for 'orphaned' child records. Once you have a lookup with each child having a parent than you can go back and convert the lookup field to a master-detail.

Thanks