Structural properties of pristine and defective graphene nanoribbons have been investigated by stretching them under 5 percent and 10 percent uniaxial strain until the nanoribbons fracture. The stretching process have been carried out by performing molecular dynamics simulations at 1 K and 300 K to determine the temperature effect on the structure of the graphene nanoribbons. Results of the simulations indicated that the conformation of the initial graphene nanoribbon model, temperature, and stretching speed have a considerable effect on the structural properties, however they have a slight effect on the strain value. The maximum strain at which fracture occurs is found to be 46.41 percent for zigzag 8 layer pristine graphene nanoribbon at 1 K and fast stretching process. On the other hand, the defect formation energy is strongly affected from temperature and nanoribbon type. Stone-Wales formation energy is calculated to be 1.60 eV at 1 K whereas 30.13 eV at 300 K for armchair graphene nanoribbon.