The purpose of this study was to examine the effects of a metaphorical interface on germane cognitive load in Web-based instruction. Based on cognitive load theory, germane cognitive load is a cognitive investment for schema construction and automation. A new instrument developed in a previous study was used to measure students' mental activities of schema construction and automation supported by structural cues in a metaphorical interface environment. Eighty participants were randomly assigned to one of two types of instructional units with the same instructional content and different interface types (i.e., non-metaphorical interface and metaphorical interface). The results indicated that germane cognitive load positively affected learning performance while there was no relationship between germane cognitive load and students' prior knowledge. A metaphorical interface enhanced learners' germane cognitive load and learning performance, and both germane cognitive load and prior knowledge similarly contributed to learning performance. The findings provide implications for the advancement of cognitive load theory and the practice of instructional development.