The \(\psi\) fractional integral is a product of the combination of Riemann-Liouville fractional integral and Stieltjes integral. The integral transforms including Laplace transform and Mellin transform for this fractional integral are introduced. The mapping properties of \(\psi\) fractional integral are mentioned. The reason that the condition of \(\psi (t)\rightarrow +\infty\) as \(t\rightarrow +\infty\) must be imposed in the definitions of \(\psi\) fractional integral and derivatives is explained when they are applied to \(\psi\) fractional differential equations.