Grain boundary (GB) plays a key role in determining the electrical and mechanical properties of mono-layer transition metal dichalcogenide (TMDC), however it is still a challenge to uncover the GB-mediated TMDC material experimentally. In this paper, the effect of twin boundary on the tensile behaviors of mono-layer MoS ₂is investigated by using the molecular dynamics simulation combined with the Stillinger-Weber potential. Mono-layer MoS ₂model under the varied size and temperature condition is adopted. Stress calculation is performed by using Virial theorem. The results are obtained as follows. 1) Twin boundary promotes the brittle fracture of an undefected mono-layer MoS ₂sheet by inducing the nucleation of the crack near boundaries, thus the fracture strength and strain are weakened. 2) Increasing the ambient temperature from 1 K to 600 K, the crack nucleation process near the twin boundary is intensely accelerated, and the fracture strength and strain are further declined. 3) Twin lamellar spacing also plays an important role in the tensile process of mono-layer MoS ₂, and the specimen with dense twin boundary, especially with void, shows higher fracture strain. 4) Stress analysis at an atomic level outlines the stress concentration caused by voids and the shielding effect of twin boundary. Because of the interactions between voids and twin boundary, the fracture strength and strain of a voided mono-layer MoS ₂sheet can be greatly improved.