This study intends to explore the influence of partners’proficiency levels on test-takers’performances in a paired oral assessment, and tries to analyze some underlying causes for the influence on the basis of Vygotsky’s theory of the zone of proximal development （ZPD）. An experiment including three rounds of tests was carried out with 12 sophomores selected from the University of Science and Technology of China. The findings showed that the high-level test-takers received the highest scores when paired with middle-level partners, the middle-level test-takers got the highest scores when working with high-level partners and got the lowest scores with low-level partners, while the low-level test-takers had basically the same scores when cooperating with partners of the three different levels. And the increase of scores was mainly due to corresponding increase of the amount of words and lexical diversity produced in the paired oral test. The results could be partly explained by the fact that the more knowledgeable other students stimulated the lower level test-takers to the upper limit of the ZPD through the language interaction. On the contrary, the mechanism of ZPD might not work when the partners’proficiency levels were much too higher than the ceiling of the test-takers’ZPD.