Boron nitride nanotubes (BNNTs) are promising for mechanical applications owing to the high modulus, high strength, and inert chemical nature. However, up to now, precise evaluation of their elastic properties and their relation to defects have not been experimentally established. Herein, the intrinsic elastic modulus of BNNTs and its dependence on intrinsic and deliberately irradiation-induced extrinsic defects have been studied via an electric-field-induced high-order resonance technique inside a high-resolution transmission electron microscope (HRTEM). Resonances up to fourth order for normal modes and third order for parametric modes have been initiated in the cantilevered tubes, and the recorded frequencies are well consistent with the theoretical calculations with a discrepancy of ���1%. The elastic moduli of the BNNTs measured from high-order resonance is about 906.2 GPa on average, with a standard deviation of 9.3%, which is found to be closely related to the intrinsic defect as cavities in the nanotube walls. Furthermore, electron irradiation in HRTEM has been used to study the effects of defects to elastic moduli and to evaluate the radiation resistance of the BNNTs. Along with an increase in the irradiation dose, the outer diameter has linearly reduced due to the knock-on effects. A defective shell with nearly constant thickness has been formed on the outer surface, and as a result, the elastic modulus decreases gradually to ���662.9 GPa, which is still 3 times that of steel. Excellent intrinsic elastic properties and decent radiation-resistance prove that BNNTs could be a material of choice for applications in extreme environments, such as those existing in space.