This paper aims at describing the mechanism of German loanwords adaptation with respect to constraints of Persian language and within OT framework. Consequently the adaptation of consonant clusters and diphthongs as well as the phonemes substituted in loanwords will be examined. Prince and Smolensky’s (1993) Optimality Theory with its key notions of faithfulness and markedness constraints is suited to model this aspect of linguistic competence. So in this research a number of 30 German loanwords were selected as research data of which some were collected through the library method from written resources and the rest are the trade names of German Products that are collected through a field work. Descriptive analysis of the mentioned data within Optimality Theory comes into valuable linguistic conclusions such as: “In Persian, initial consonant clusters of German loanwords are broken up through vowel epenthesis which is mostly identical to the vowel of the second syllable.”
