The paper aims to focuses on the Morphological complexity in Tamil language from the point of view of POS tagging. Nouns get inflected for number and cases. Verbs get inflected for various inflections which include tense, finite and non-finite suffixes. Verbs are adjectivalized and adverbialized. Also verbs and adjectives are nominalized by means of certain nominalizers. Adjectives and adverbs do not inflect. Many post-positions in Tamil are from nominal and verbal sources. So, many times we need to depend on syntactic function or context to decide upon whether one is a noun or adjective or adverb or post position. This leads to the complexity of Tamil in POS tagging.