The paper focuses on the rule based case transfer, which is a part of the transfer grammar module developed for bidirectional Tamil to Malayalam Machine Translation system. The present study involves two typologically close and genetically related languages, namely Tamil and Malayalam. We considered the basic construction of sentences which is highly dependent on the case systems. The rules were written by taking into consideration the Postpositions and cases in the languages. A parallel corpora was chosen and a deep analysis of the case transfer patterns were done and rules were written to sort out the case changes that happens when translating from one language to another. We have also considered copula transfer in our approach. Web data was used for evaluation and the results were encouraging.