<p>Hexagonal boron nitride (hBN) nanosheets have become the most promising candidates as gate dielectric and insulating substrates for two-dimensional (2D) material-based electronic and optoelectronic devices. While mechanical stress in hBN nanosheets is often either intrinsically or intentionally introduced for 2D material-based devices during device fabrication and operation, the dielectric strength of hBN nanosheets under mechanical stress is still elusive. In this work, the dielectric strength of hBN nanosheets in a metal/hBN/metal structure is systematically studied when mechanical stress normal to nanosheets is applied. The dielectric strength of hBN nanosheets is found to be weakened with lower breakdown strength, shorter breakdown time, and larger leakage current under the mechanical stress with the order of 100 MPa, and the weakening is more remarkable for thinner nanosheets. The thickness-dependent weakening of dielectric strength is attributed to the thickness-dependent stress gradient in hBN nanosheets. Furthermore, the ability of hBN nanosheets to block leakage current can be significantly degraded by mechanical stress even for thick nanosheets up to 41.3 nm. The results indicate that it is highly important to eliminate mechanical stress in high-performance 2D material-based devices employing hBN nanosheets as 2D insulators.</p>