Abstract. Natural Language Processing (NLP) is a well-known technique of artificial intelligence to extract the elements of concerns from raw plain text information. It can be utilized to process the early software requirements in order to achieve the goals like requirement prioritization and classification (functional and non-functional). To the best of our knowledge, no research work is available yet to examine and summarize the utilization of NLP in the domain of Software Requirement Engineering (SRE). Therefore, in this paper, we investigate the applications of NLP in the context of SRE. A Systematic Literature Review (SLR) is carried out to select 27 studies published during 2002-2016. Consequently, 6 NLP techniques and 14 existing tools are identified. Furthermore, 9 tools and 2 algorithms, proposed by the researchers, are presented. It has been concluded that the NLP techniques and tools are highly supportive to accelerate the SRE process. However, some manual operations are still required on initial plain text software requirements before applying the desired NLP techniques.