In this paper we present the 13C relaxation measurements of fluids saturated in oil reservoir cores under the condition of magic angle spinning (MAS). The longitudinal relaxation of the fluids generally shows a biexponential process, which can be decomposed as a slower one plus a faster one. These two different processes are attributed to the motion of molecules which were distributed in macro- and micropores, respectively, the exchange of which is restricted. We therefore could measure the fraction of molecules in macropores and micropores in the core samples by relaxation decomposition. For molecules remaining in the pores, a very slow magic angle spinning rate can efficiently eliminate the line broadening due to inhomogeneous susceptibility. Measurements of various MAS rates and the spinning rate dependence of the relaxation behaviors are illustrated. A reasonable explanation is made based on the increase of exchange rate between molecules in different pores due to the sample spinning.