Background: Children usually produce their first words when they are 10-15 months old and go through simple to complex speech by passing some stages. One of the criteria for examining development of children's language is mean length of utterance (MLU). The main purpose of this study was calculating mean length of utterance and grammatical morphemes used by Farsi-speaking children during their 12-60th months of life.Case: It was a longitudinal descriptive study. Every month during children's 12 to 60th months of life, 120-minute spontaneous speech samples of two children in kindergarten were videotaped and transcribed. The girl said her first word at the age of 12, and the boy said his first word at the age of 16 months. Combining words and constructing two-word utterances started at 18 and 20th months of the girl's and the boy's lives respectively. First grammatical morpheme appeared before the 24th month of children's lives and when mean length of utterance was lower than 2 morphemes. Singular verb suffixes were acquired sooner than the plural ones. Both children started using six subject identifiers of Farsi language before 36th month of their lives.Conclusion: The speech development in Farsi-speaking children follows the same pattern as other children, starting from one-word utterances and complicates gradually by increasing the number of words, word combinations and using grammatical morphemes. The important point is that Farsi-speaking children started to use grammatical morphemes when their mean length of utterance was lower than two morphemes.