All ferroelectrics exhibit piezoelectricity. It is interesting to know the piezo-response of our FE MOX 2 monolayers as strong in-plane piezoelectricity has already been predicted in 2D FE group-IV monochalcogenides . Our calculated piezoelectric stress coecients (e ij ) are shown in Table . e ij are important coecients for estimating gure-of-merit of a piezoelectric thin-lms; usually larger the e ij , higher the gure-of-merit. Because strain along z-direction (vacuum) are ill-dened in 2D materials, we have only three independent piezoelectric coecients: e 11 , e 12 , and e 16 . There is a mirror symmetry along the b -direction, which does not allow any polarization in that direction, thus e 22 = 0. However, strain along b -direction can induce polarization along adirection, which results e 12 coecient. FE MOX 2 monolayer (space group: Pmm2) due to point group mm2 has a symmetry of reection with reference to M-O atomic plane. This prohibits an out-of-plane electric polarization, thus e 31 = 0. We mainly focus on the piezo-response related to uniaxial strain along the a -direction (η 1 ) and b -direction (η 2 ), which are e 11 (d 11 ) and e 12 (d 12 ), respectively. e 16 is associated with shear strain (η 12 ) , and we exclude it for simplicity.