A custom object contains some records, now my requirement is to create field in this object with master detail relationship. Can we create master detail relationship in this case?

No, directly we cannot create master details relationship if custom object contains existing records.

   Following are the steps to create to create master-detail relationship when records are available in custom object.

1. First create field with lookup relationship.
2. And then associate look field with parent record for every record
3. Next change the data type of the field from look up to Master detail.