Spatial skills are positively related to mathematical abilities, yet the mechanism of such relation remains unclear. This study examined the mediating role of mental number line representation in the relations between spatial skills and performance in various mathematical tasks. One hundred and nine second-graders were tested on mental rotation, perspective taking, mental number line representation, visuospatial working memory, arithmetic fact retrieval, calculation and word problems. Using structural equation modelling with the bootstrap procedure, we found that after controlling for age, gender and visuospatial working memory, mental number line representation fully mediated the relation between spatial skills and calculation and the relation between spatial skills and word-problem solving. However, it was not a significant mediator between spatial skills and arithmetic fact retrieval. This study highlights the important role of mental number line representation in explaining the relation between spatial skills and mathematical abilities. Copyright © 2018 Elsevier Inc. All rights reserved.