Mediation in general, and teacher and peer mediation in particular, whose genesis are Vygotsky's sociocultural and Feuerstein's mediated learning experience theories, is a novel approach aimed at teaching low level learners by an experienced instructor or peer. The main objectives of the present study, however, were to investigate the effects of teacher and peer mediation on EFL learners' grammar learning (active vs. passive voice) at pre-intermediate level. In addition, the focus was on the effectiveness of two strategies of mediation on the grammar learning and retention of target forms. To this end, a sample of 45 EFL learners at the age range of 14 to 19 was selected from among 68 low-intermediate learners of a private language institute in Khoy, Iran. A homogeneity test, pretest, immediate and delayed posttests were used as data collection tools. The results revealed that teacher mediation was more effective than the peer mediation and traditional group in improving the learning of active vs. passive voice in both the short-run and the long-run.