Ribosomal repeats occupy 5% of a plant genome, yet there has been little study of their diversity in the modern age of genomics. Ribosomal copy number and expression variations present an opportunity to tap a novel source of diversity. In the present study, we estimated the ribosomal DNA copy (rDNA) number and ribosomal RNA (rRNA) expression for a population of maize inbred lines, and investigated the potential role of rDNA and rRNA dosage in regulating global gene expression. Extensive variations were found in both ribosomal DNA copy number and ribosomal RNA expression among maize inbred lines. However, rRNA abundance was not consistent with the copy number of the rDNA. We have not found that the rDNA gene dosage has regulatory role in gene expression; however, thousands of genes are identified to be co-regulated with rRNA expression, including genes participating in ribosome biogenesis and other functionally relevant pathways. We further investigated the potential roles of copy number and the expression level of rDNA on agronomic traits, and found that both correlated with flowering time, but through different regulatory mechanisms. This comprehensive analysis suggested that rRNA expression variation is a valuable source of functional diversity that affects gene expression variation and field-based phenotypic changes.