Herein, it is determined that the stress response of stacked microlamellae to tensile deformation along the normal direction depends on the lateral size of the lamellae. Lamellae with different lengths are formed by B5‐29‐EMA‐φ block copolymers (BCPs) comprising poly(ethyl methacrylate) (PEMA) segments attached to a liquid crystal (LC) BB‐5(3‐Me) polyester having Mn = 29 kDa at both ends at PEMA volume fractions of φ%. The LC and PEMA blocks are segregated to form microlamellae; however, the lamellar length depends on the value of φ; B5‐29‐EMA‐22 forms long‐range lamellae, and B5‐29‐EMA‐44 comprises LC block lamellae separated by PEMA blocks. The stress response is similar between these copolymers in the elastic region but differs in the succeeding stage; B5‐29‐EMA‐22 films show a stress plateau, and B5‐29‐EMA‐44 films continuously increase the stress, displaying the strain hardening. Small‐angle X‐ray scattering measured simultaneously with the stress–elongation curves discloses that these different stress responses accompany individual lamellar deformations. B5‐29‐EMA‐22 undulates and folds the lamellae, whereas B5‐29‐EMA‐44 maintains parallel lamellar stacking, remarkably increasing the PEMA block lamellar thickness. These preferentially thickened lamellae are occupied by extended PEMA chains, which causes stress at the expense of the chain entropy, resulting in strain hardening.