【Abstract】Both first language acquisition and second language acquisition are the base of studying language teaching and learning. Discuss on similarities and differences between them from linguistic， cognitive and social perspectives gives a better understanding of first and second language acquisition. This essay illustrates the similarities and differences and discuss their pedagogical significance on the basis of teachers’ understanding and experience.
　　【Key words】first language acquisition； second language acquisition； similarities； differences
　　First language acquisition （FLA）， literally， is relevant to the acquisition of the mother tongue （Ellis， 1997）. SLA， short for Second language acquisition， refers to the study of the acquisition of the additional language subsequent to learning the mother tongue in different circumstances （Ellis， 1997）. Having been researched for over 40 years by many cross-disciplinary scholars such as Chomsky and Krashen， the study of FLA and SLA contributes to pedagogical development considerably. To make a comparison and contrast between FLA and SLA， a clear picture of both studies which will help in improving pedagogy prospect can be drawn. This essay will illustrate the similarities and differences between FLA and SLA from linguistic， cognitive and social perspectives and discuss their pedagogical significance on the basis of teachers’ understanding and experience.
　　From the linguistic perspective， the expected leaning stages and the linguistic competence in the final state are comparable between FLA and SLA. As Saville-Troike （2006） writes， for each of the L1 and L2 learner， the language of lexicon， phonology， morphology， syntax and discourse is expected to be applied essentially. Whereas， there is a big difference in linguistic competence， which is defined as the outcome of language learning achieved by L1 and L2 learners in real situation. According to Saville-Troike （2006）， the L1 learners can obtain the underlying knowledge of the mother tongue unconsciously with no difficulties. Some believe that Universal Grammar， which is proposed by Chomsky， facilitates the mastery of the underlying knowledge of the mother tongue （Mitchell and Myles， 2004）. Conversely， Saville-Troike also points out that L2 learners seldom reach the high level of competence on account of the fossilization in certain level. In the process of acquiring the second language， learners seem to make the same mistakes repeatedly or have a certain accent （Littlewood， 1991）. In general， fossilization is an inevitable phenomenon during the process of L2 acquisition no matter how much effort have been made for most of the learners. From pedagogical perspective， teachers can help students get over the fossilization by giving corrective feedback when some mistakes are made continually based on the analysis of the students’ different learning features and the variety types of fossilization. Overall，， analysed from the linguistic aspects， the most similar feature is the necessity of the same linguistic stages for FLA and SLA and the linguistic competence differs between L1 and L2 learners.