In recent years, despite the fact that many researchers have devoted much of their attention to second language attrition, not much focus has been given to first language attrition (FLA) specifically among Iranian immigrants. The present study attempts to describe FLA in the semantic domain of idiomatic expression and effect of length of residence among Persian native speakers who live in Iran as well as those who migrate to English-speaking countries. The present study explores language attrition in three migrant populations (Persians in the United States, Australia, and Canada). The participants were selected through convenience sampling. Furthermore, to find the impact of length of residence, the immigrants were divided into two groups comprising short- and long-term residence groups. The instrument applied by the researchers for data collection included a researcher-devised idiomatic expression test to assess immigrants' level of idiom comprehension and demographic information questionnaire to have a better understanding of immigrants' background characteristics. Results revealed that the immigrants underwent FLA and the rate of attrition was higher in long-term immigrants. The results are in harmony with the "Activation Threshold Hypothesis" showing the language attrition among fewer L1 users. The finding of this study sheds new light on the understanding of the concept of first language attrition in migration studies.