5-Methylcytosine (m.sup.5C) is a type of chemical modification on the nucleotides and is widespread in both DNA and RNA. Although the DNA m.sup.5C has been extensively studied over the past years, the distribution and biological function of RNA m.sup.5C still remain to be elucidated. Here, I explored the profiles of RNA m.sup.5C in four mouse tissues by applying a RNA cytosine methylation data analysis tool to public mouse RNA m.sup.5C data. I found that the methylation rates of cytosine were the same with the averages of methylation level at single-nucleotide level. Furthermore, I gave a mathematical formula to describe the observed relationship and analyzed it deeply. The sufficient necessary condition for the given formula suggests that the methylation levels at most m.sup.5C sites are the same in four mouse tissues. Therefore, I proposed a hypothesis that the m.sup.5C formation catalyzed by RNA methyltransferase is random and with the same probability at most m.sup.5C sites, which is the methylation rate of cytosine. My hypothesis can be used to explain the observed profiles of RNA m.sup.5C in four mouse tissues and will be benefit to future studies of the distribution and biological function of RNA m.sup.5C in mammals.