Calcium silicate hydrate (C-S-H), the essential binder of cement based materials, is a poor crystal phase at nano-scale. In this study, crack growth mechanism is unraveled in lights of Molecular Dynamics (MD) by simulating the uniaxial tension test on the C-S-H gel with voids ranging from 0.5. nm to 5. nm. At nano-scale, the layered C-S-H gel demonstrates dual nature of crack propagation. In xy plane, the stable ionic-covalent bonds Si-O and Ca-O are hard to break so the cracks coalesce is slowed down, implying ductile characteristic. In z direction, cracks spread in the interlayer region with high rate due to the frequently breakage H-bonds network, exhibiting brittle nature. In the tensile process, the crack develops from the central void and "strain concentration" near the void boundary is induced by irreversible atomic deformation. Young's modulus and tensile strength of C-S-H gel are significantly weakened due to the presence of central void. Additionally, due to the binding constraints' discrepancy, bending of the calcium silicate sheet can be observed, reflecting the complicated tensile behavior of heterogeneous layered C-S-H gel.