Extending studies on emotional design in multimedia learning beyond college samples and further considering individual differences, we examined the effects of emotional design on middle school students' learning and whether this relationship is moderated by learners' prior knowledge, using a computer-based lesson covering the topic "the formation of lightning." Participants were middle school students (N=124; 58 females; age range 13 to 16) who volunteered to participate with permission. Learners with high prior knowledge (n=63) and learners with low prior knowledge (n=61) were randomly assigned to one of two instructional design conditions: positive emotional design (colourful and anthropomorphic features) and neutral emotional design (grey and non-anthropomorphic features). The results showed that the positive emotional design operationalised via visual elements did not induce more positive emotions than the neutral design group, although there was a trend towards facilitating more learning transfer ([eta][superscript 2][subscript p]=0.02, p=0.098) and increasing the mental effort of learners with low prior knowledge ([eta][superscript 2][subscript p]=0.03, p =0.06). There was a tendency for learners with high knowledge to perceive higher mental effort ([eta][superscript 2][subscript p]=0.03, p=0.06), satisfaction ([eta][superscript 2][subscript p]=0.05, p=0.01) and achieve better transfer scores ([eta][superscript 2][subscript p]=0.03, p=0.087) compared with those learners with low knowledge. These findings suggest that positive emotional design should be cautiously applied in multimedia learning for middle school students. Individual differences must be considered in instructional design in a multimedia learning environment.