In this paper we study the Riemann-Liouville fractional integral of order α > 0 as a linear operator from L p (I, X) into itself, when 1 ≤ p ≤ ∞, I = [t 0 , t 1 ] (orand X is a Banach space. In particular, when I = [t 0 , t 1 ], besides proving that this linear operator is bounded, we obtain necessary and sufficient conditions to ensure its compactness. We also prove that Riemann-Liouville fractional integral defines a C 0 −semigroup but does not defines a uniformly continuous semigroup. We close this study by presenting lower and higher bounds to the norm of this operator.