Relationship between precipitation strengthening and creep resistance improvement has been an important topic for the widespread applications of magnesium alloys. Generally, static precipitation strengthening through thermal stable precipitates would generate satisfactory creep resistance. However, an opposite example is presented in this work and we propose that the size of precipitates plays a crucial role in controlling the operative creep mechanisms. In addition, the precipitate components along with their crystal structures in the crept Mg–4Al–3Sm–0.4Mn samples with/without pre-aging were thoroughly studied using Cs aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Previous aging generates a large density of fine precipitates (< ∼5 nm) homogeneously distributing in Mg matrix and exhibiting satisfactory strengthening effect. However, the number density of precipitate strings consisting of several or even dozens of relatively coarse precipitates (∼10 nm) was significantly decreased at the same time. As revealed in this work, the relatively coarse particles in Mg matrix are much more efficient than the fine precipitates in promoting dislocation climb. Therefore, the rate-controlling mechanisms are transferred from dislocation climb to dislocation slip after previous aging, thus leading to degradation of creep resistance. Moreover, there are mainly five types of precipitates/clusters, namely β´´-(Al, Mg) ₃Sm, Al ₅Sm ₃, ordered Al–Sm cluster, ordered Al–Mn cluster and ordered/unordered AlMnSm clusters. The crystal structures of the former two precipitates were discussed and the formation mechanisms of the precipitates/clusters were revealed.