The term critical period refers to a critical phase of life between the age of two and before beginning of the puberty when each individual is able to acquire the first language. After this critical period the acquisition of the first language is far more difficult and will never reach the perfect status of acquired language. On the other hand Critical Period Hypothesis aims to investigate the reasons between first language acquisition and second language acquisition. Many researchers believe that there is a critical period in each human's development in which second language learning may occur to certain level that the individual may become able to speak the additional language as a native speaker. It is believed that after the period of puberty it is nearly impossible for each individual to acquire the second language to a higher degree of proficiency.