Dynamic Assessment (DA), based on Vygotsky's concept of Zone of Proximal Development, has at its core the integration of assessment and instruction through mediation. DA is practiced either individually or collectively. In group-dynamic assessment(G-DA), as Poehner (2009) introduced, learners are simultaneously exposed to mediation. The problem which has been ignored in G-DA is the homogeneity and heterogeneity of EFL leaners while being mediated. This study was an attempt to investigate the effect of group-dynamic assessment on homogeneous and heterogeneous EFL learners' listening comprehension ability. It also intended to find out if expert-novice and peer-to-peer mediation types in G-DA differed in their effects on homogeneous and heterogeneous EFL learners' command of listening comprehension. To this end, eighty intermediate EFL learners were chosen to be the participants. According to their language proficiency, they were assigned to two groups of forty homogeneous and forty heterogeneous learners. Then, each group was divided into two experimental groups. The participants in all groups attended three listening sessions. For the mediation, the participants were assisted through novice-expert and peer-to-peer mediation types. To find out the main and interaction effect of group-type and mediation-type in G-DA, the post-test scores were analysed through running a two-way ANOVA. The results indicated that both group-type and mediation type had significant main and interaction effect on learners' listening comprehension. Also a one-way ANOVA proved that mediation- type had more significant effect on heterogeneous rather than homogeneous EFL learners.
Key words: Group- Dynamic Assessment, homogeneous, heterogeneous, mediation type, Zone of Proximal Development,