We have used reverse nonequlibrium molecular dynamics modeling to study the impact of uniaxial stretching on the thermal conductivity of monolayer molybdenum disulfide (MoS2) and hexagonal boron nitride (h-BN). Our results predict an anomalous response of the thermal conductivity of these materials to normal strain. Thermal conductivity of h-BN increases under a tensile strain whereas thermal conductivity of MoS2 remains fairly constant. These are in striking contrast to the impact of tensile strain on the thermal conductivity of three dimensional materials whose thermal conductivity decreases under tensile strain. We investigate the mechanism responsible for this unexpected behavior by studying the impact of tensile strain on the phonon dispersion curves and group velocities of these materials.