Uniaxial deformation at finite strain of coalesced core/shell (hard core/film-forming shell) latex films is investigated by means of micromechanical calculations. Elongation ratios, strain rate, and energy density distribution within the film are presented and confirm the strain amplification phenomenon well-known in the filled elastomer area. The important role of the core-shell interphase on the overall film mechanical behavior is stressed by the presented results. Moreover, strain-stress curves have been calculated without adjustable parameters and compared to experimental ones in order to gain substantial information about the deformation mechanism. It is then proposed that uniaxial deformation of coalesced core/shell latex films proceed through two simultaneous and/or successive mechanisms: isotropic matrix deformation and geometrical core rearrangement within the film. The stiffening of coalesced core/shell latex films appears therefore to be mainly due to mechanical effects.