Carbon nanotubes(CNTs)/two-dimensional transition metal dichalcogenides(MX2)hybrids have shown unique physical properties,making them promising materials for various applications ranging from photocatalysis to solar energy conversion.Understanding the interfacial interactions is highly desirable for designing these hybrids having excellent performance.Here we systematically study the interfacial interaction in single-wall CNT/monolayer MoS2 hybrids and its effects on electronic and optical properties by first-principles calculations.It is found that CNT is interacted non-covalently with monolayer MoS2 forming van der Waals heterostructures,and their interfacial interaction is closely related to tube diameter.In these hybrids,interestingly,MoS2 gaining or losing electrons depends also on tube diameter:(3,0)CNT would gain some electrons from MoS2,whereas other CNTs lose some electrons.The small band gap makes these hybrids having a strong optical absorption in visible light region.The type-Ⅱ,staggered,band alignment in CNT/MoS2 hybrids can facilitate the separation of photoexcited electrons and holes,improving the photocatalytic activity.Moreover,the CNTs are not only an effective sensitizer,but also a highly active co-catalyst in hybrids.These results have revealed the mechanism of enhanced photocatalytic performance of CNT/MoS2 hybrids observed in experiments,and help for developing highly efficient carbon nanomaterial-based nanophotocatalysts.