The relationship between identity processing styles and language proficiency in English as foreign language (EFL) was investigated among the Persian EFL learners. 266 Persian candidates taking part in a Ph.D. examination at Shiraz University took part. The Language Proficiency Test was used to measure language proficiency in English. The Identity Styles Inventory was used to measure normative, informational, and diffuse-avoidant identity processing styles. Relationships between normative and informational styles and language proficiency and its subscales (grammar, vocabulary, and reading) were positive and significant. Negative relationships between diffuse-avoidant style and language proficiency and its subscales (grammar, vocabulary, and reading) were observed. There were significant sex differences for diffuse-avoidant style and for vocabulary.