abstract this research is about a longitudinal case study of english morpheme acquisition by a persian speaking child l2 learner of english (2 .9-3).the goal of this research has been discovery of the child ‘s ability in acquiring of english morphemes in a persian context while only one person (the child’ s father)has been talking to her.this child has also been exposed to english language from the age of one.her output has been collected from the age of 2.9 to 3 to examine her progress.the hypotheses are: 1-the process of lexical morpheme acquisition in this child is like the mother language process but slower.2-the process of inflectional suffix acquistion in this child is as same asthe mother language but slower.this research is a longitudinal case study.the data collection has been done through recording the childs utterances ,taking film of her and mostly writing her english output in the notebook.the data has been gathered in different situations such as the time of her playing with herself or with her partners or the time of her talking to her fatherand other places.the child,s output has been analyzed by means of tables and figures.the results of this research are:1-correlation of this work with browns results and his two stages of language acquisition.2- correlation of this childs grammatical morphemes with work of duley and burt,crashen and other scientists.3-finding evidences of two systems of acquisition in this child.4-aggreement with crashens contextual effect.5-proving of two hypotheses of this work which are likeness of lexical morpheme acquisition of mother language and second language of this child but the latter one with slower progress and the same condition for inflectional suffixes.