Background: While it has been widely demonstrated that children's and adolescents' understanding of rational number plays an important role in their mathematics achievement, we have limited knowledge about the cognitive correlates of this understanding. Aims: The current study aimed at examining whether children's non-symbolic ratio processing and their understanding of place-value structure of whole numbers play a role in their understanding of fractions and decimals and whether their roles are different for fractions versus decimal understanding. Sample: A sample of 124 fourth graders was tested. Methods: Participants were tested on their symbolic rational number processing, non-symbolic ratio processing, place-value understanding of whole numbers, mathematics achievement, as well as a series of domain-general and domain-specific cognitive skills related to symbolic rational number processing. Results: The findings suggest that, while the understanding of place value of whole numbers significantly predicted the understanding of both fractions and decimals, non-symbolic ratio processing specifically predicted the understanding of fractions, but not decimals. Conclusions: The findings highlight the roles of place-value understanding and non-symbolic ratio processing in the acquisition of symbolic rational numbers.