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Recent experimental studies have shown that multi-grains are ubiquitously present in graphene grown with chemical vapor deposition method technique. The potential application of the unique two-dimensional material in future nanotechnology demands full understandings of their structure and properties. Using molecular dynamics simulations, we study the mechanical response of various bicrystal graphene consisting of symmetric tilt boundary subject to uniaxial tensile loading. Both bulk graphene and graphene nanoribbons (GNRs) are studied. We revealed that nano-crack initiated at grain boundaries (GBs) leads to brittle failure with no plasticity at room temperature. The mechanism that crack nucleated at the intersection of GB and free surface followed by fast advance of crack, preventing plasticity that involves dislocation slip or GB sliding that is seen in metals. Cleavage along GBs is observed to be dominant fracture behavior in the studied GNRs. Furthermore, the ultimate tensile strength decreases with increasing vacancy concentration, illustrating that the lowered strength of GB interface is primarily due to not well-bonded atoms, shedding light to the structure-properties relationship. Finally, local strain and atomic-level stress have been shown to be able to characterize the onset of crack nucleation and thereby good quantities for predicting the resulting ultimate tensile strength.