Weak strain overshoot (WSO) of loss modulus accompanying with strain softening under large amplitude oscillatory shear deformation is a common phenomenon that occurs in many systems but is still far from being clarified. Study about WSO has a strong practical significance, for it is directly related to sharp increase of energy dissipation of elastomer nanocomposites experiencing dynamic deformation of large strain amplitudes. Herein a systematical investigation was performed on WSO of uncured natural rubber (NR) and its low-loading carbon black compounds and a frequency-temperature-volume fraction diagram for the onset of WSO is firstly proposed to shed light on the mechanisms. A comparison of rheology behaviors of uncured polyisoprene rubber and NR and cured polyisoprene rubber suggests that WSO in NR originates from the non-isoprene fraction. Fourier-transform infrared spectroscopy measurement suggests the dissociation of hydrogen bonding at high temperatures, which accelerates the stress relaxation.