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In order to investigate the relationship between aggressiveness and oral proficiency of Iranian EFL learners, first a TOEFL test was given to 100 EFL students in order to homogenize the sample. Out of this, 71 participants whose scores fell one standard deviation above and below the mean were regarded as intermediate and, therefore, interviewed. They were then asked to complete the Persian version of a validated aggression questionnaire. All the tape-recorded interviews were rated by two raters. Based on their scores on aggression questionnaire, the subjects were divided into two groups of aggressives and non-aggressives and the means of their scores in oral interviews were compared using t-test. Results of the t-test showed that, aggressive and non aggressive groups are different in their oral proficiency. Finally, the correlations between the two main variables and also between four subscales of aggression and all the components of oral proficiency were estimated to see exactly what the nature of the relationships is. Overall, the results of these calculations showed that aggression negatively affects oral proficiency of L2 learners. Moreover, verbal aggression and anger as different subscales of aggressiveness were found to have negative effect on the components of oral proficiency.