We investigate weak lensing effect on cosmic microwave background (CMB) in the presence of a superhorizon mode which leads to a dipole anisotropy in the CMB power spectrum and in large scale structures. The approach of flat-sky approximation is considered. We determine the functions σ02 and σ22 that appear in expressions of the lensed CMB power spectrum in the presence of the dipole anisotropy. We determine the correction to B-mode power spectrum which is found to be appreciable at low multipoles (l), around 15 % at l= 10. However, the temperature and E-mode power spectrum are not altered significantly.