The excellent mechanical properties of single- and few-layer graphene have been well-quantified and evidenced by computational methods and local indentation measurements. However, there are less experimental reports on the in-plane mechanical properties of multilayer graphene sheets, despite many practical applications in flexible electronic and energy devices (e.g. graphene flexible electronic display, battery, and storage devices) are actually based on these thicker nanosheets. Here, in-plane fracture behaviors of multilayer graphene nanosheets with thicknesses between ∼10 and 300 nm (∼10-1000 layers) are characterized and quantified by in situ scanning electron microscopy and transmission electron microscopy under tensile loading. We found that, generally, the fracture strengths of graphene nanosheets decrease as the thickness (or layers) increases; however, the fracture strain of thinner graphene sheets is less than that of thicker sheets. The fracture process of the thicker nanosheets includes the initial flattened stage, the stable elastic stage, and the rapid fracture with brittle characteristics, while the thinner nanosheets show obvious delamination between the atomic layers at fracture. This work provides critical experimental insights into the tensile fracture behavior of multilayer two-dimensional materials and a better understanding on their realistic mechanical performance for potential flexible device and composite applications.