The mechanical properties of superlattice (SL) TMN (transition-metal nitrides) coatings with different as-deposited structures are often quite different. These differences in mechanical properties can be attributed to distinct deformation and strengthening mechanisms. Here, we discuss the strengthening mechanisms of single- and poly-crystalline SLs under nanoindentation loads. We observe that the dislocation behaviors during nanoindentation, such as dislocation accumulation and crossing interfaces, are responsible for the strengthening of single-crystalline SL coating, whereas no such pronounced strengthening is observed in the polycrystalline SL. We further reveal the monoclinic phase transformation occurring at the SL, solid solution zone, and crack tip region in the single-crystalline coating. Phase transformation alters the SL interface's structure, facilitating dislocation accumulation. Consequently, it raises the theoretical yield stress of single-crystalline coating. For polycrystalline coating, we observed a localized monoclinic phase present only near the crack tip. The current research unravels TMN SL strengthening mechanism at the atomic scale.