Given the changing nature of literacy, there is an urgent need to develop alternative ways of assessment in support of students' new literacy practices in the digital age. While emergent models of multimodal assessment are being developed in theoretical contexts, the study reported in this paper illustrates how multimodal theories can be realized in classroom practice. Seeking to address the needs of both literacy instruction and assessment, this study proposed a design rubric from the multiliteracies perspective to assess students' design of multimodal texts as a means to support assessable new literacy practices. Specifically, this research aimed to investigate how the design rubric as a formative assessment tool affects English learners' multimodal text production (in this case, presentation slides). The empirical results of this study reveal that the theory-driven design rubric was useful for enhancing the students' understanding and awareness of the multimodal nature of presentation slides and led to improvement in their multimodal text production. The findings have important implications for scaffolding students' multimodal literacy by using formative assessment as one of the instructional approaches in multiliteracies pedagogy. (Contains 5 tables and 1 figure.)