Letters are regarded as one of the most important sources of our information on Mesopotamian civilization because they represent the real situation of the Mesopotamia society. Therefore, these letters occupy most of the researchers attention. This research is an attempt to pin-point the similarities and differences in the form and style of letters writing since the Summerian eras and the successive new Babylonian era. The research deals with some methods used for communication before the inventory of writing and the way of selecting messengers to achieve this vital role. In addition to that it studies the forms of the letters and any changes made on these forms whether these changes were grammatical changes or formula changes and an attempt to search for origin of relations and similarities of the forms of these letters during the different eras of the Mesopotamian history. The research also tackles the study of different kinds of letters which are put in three types: secret, public, and fast letters and to show some reasons and motivations behind the appearance of these kinds of letters.