Millions of users worldwide nowadays use Machine Translation (MT) systems such as Google Translate (GT). The present study aimed at investigating the effects of using GT in class activities on the vocabulary development and phraseology of Iranian ESP learners. To this end, after assuring the homogeneity of the students, 60 ESP learners majoring in Persian Language and Literature and Physical Education and Sports Sciences were selected and randomly assigned to two experimental groups. After administering the pretest, the students in the first experimental group (Only-Dictionary) used available dictionaries but those in the second experimental group (Dictionary plus Google Translate) employed both GT and available dictionaries. As for homework, the participants were asked to translate a passage from their textbook for the following session. The students were given feedback on their translations so that they could understand their weaknesses and strong points. Ultimately, the posttest was administered which indicated that both groups improved in their vocabulary knowledge and equivalent-finding skill. However, the mean score for the Dictionary plus Google Translate group students was significantly higher than that of the Only-Dictionary group. As for phraseology, there was not a significant difference between the two groups. تفاصيل المقالة