This paper presents the first investigation of the errors in the mother tongue of the children with autism, and compare between these errors and the errors in the production of Arab learners of English as second/foreign language. Based on the previous literature, the current study noted that language delay and the impaired social skills in the children with autism are the most prominent reasons which resulted in their impaired skills of language. In addition to, primary reasons affected the production of the second /foreign language such as the delayed learning of the target language, and the lack of interaction with the native speakers in social settings. Overall, these similar reasons between the two groups led to similar errors in the productions of their native and target languages. Aims: The primary goal of the present study is to explore the similarities between the performance of the children with autism when they use their mother tongue, and the errors in the production of Arab learners of English as second /foreign language when they use the target language. Methods & Procedures: The sample of this study consisted of two Yemeni children with high functioning autism aged 7;6 and 8;3 years. The examiner used the expressive language task to elicit the morpho-syntactic structures from the two cases, and compared it to the previous literature of Arab learners of English as second /foreign language, to find out the similarities between the errors in their productions. Results: The findings of this study revealed the similarities between the errors which occur in the answers of the autistic children when they use their mother tongue, and the errors which occur in the production of Arab learners of English as second /foreign language due to their delayed learning of the native/target languages. These errors classified into omission, substitution, or deletion of grammatical structures explored in this study and included (adjectives, preposition, articles, subject-verb agreement and plurality). The children with autism and the learner of English as second/foreign language substituted the order of the adjectives at the sentence level, omitted and overused the articles, substituted the prepositions with others, replacing the feminine suffixes with masculine suffixes in the production of the autistic children, and omitting the 3<sup>rd</sup> person singular in the production of the ESL/EFL learners resulted in subject-verb disagreement, and impaired plurality by replacing the singular nouns with plural nouns in the learners of English as second/foreign language or the opposite in the children with autism.