Lexicons play a central role in natural language processing (NLP) systems, so design and implementing computational lexicons are of foundamental activities in development of such systems. In this paper first, we define lexicon, introduce its parts, study the difference factors in lexicons and describe their various types. Then after discussing the theoretical aspects design and implementation of lexicons, We will study a practical instance in a Persian natural language processing system. In this system, we will consider the mentioned theretical parameters and factors and initiate them practically.