This study explored whether achievement in English as a foreign language (EFL) is significantly related to teacher effectiveness and personality. The administration of a bio questionnaire, the Characteristics of Effective English Language Teachers (CEELT) and the NEO Five Factor Inventory (NEO-FFI) in Persian to one thousand two hundred and sixty EFL learners and one hundred eighteen EFL teachers in Mashhad, Iran, showed that EFL achievement is significantly related not only to the CEELT as a measure of teacher effectiveness but also to its Rapport, Fairness, Qualification, and Facilitation factors. Similarly, it correlated significantly with the NEO-FFI as a measure of personality. However, out of five personality dimensions only Conscientiousness, and Extraversion correlated with the EFL achievement. The classification of the EFL learners into high, middle and low achievers on the basis of their z-scores and correlating the achievement scores with the two measures showed significant relationships only for middle achievers. The various types of significant relationships between the factors underlying the CEELT and the EFL achievement for the three groups of achievers on the one hand and the significant relationships between the EFL achievement and dimensions of NEO-FFI on the other are presented and discussed. Suggestions are also made for future research.

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