The flexural/twist anisotropy, present in most balanced, symmetric laminates with angle play layers, have an important influence on elastic instability of plate composite. The maximal compression load can be reduced up to 13% in comparison with the orthotropic plate. The anisotropy changes also the deformation mode form. Based on the finite element studies, we developed an empirical law – the relation between the knockdown factor, the anisotropy factor and the plate geometry. The knockdown factor is defined as the ratio between the maximum loads of the anisotropic plate and of the orthotropic one. In this way, the semi-analytic instability calculation approach is based on analytical linear bifurcation theory with orthotropic properties and on empirical consideration of the flexural/twist anisotropy effect by the knockdown factor.