The reactivity of commercial Light Water Reactors (LWR) is given by the reactimeter, which computes the core reactivity from the flux variation measurement. Reactivity worth are measured in experimental reactors by the Doubling Time technique. In both cases, the reactivities are computed from the kinetics equations based on delayed neutron (DN) data (beta i, lambdai). Currently, there is no direct experimental validation of the relationship between the measured flux variation and the core reactivity. The uncertainty on the reactivity, obtained by propagation of the uncertainty of the DN data, amounts to more or less 6percent (1sigma) mainly linked to the DN average lifetime. This paper presents the direct validation of the kinetics relationship between the reactivity and the flux variation through EOLE integral experiments. This work allows the bias calibration of the deduced LWR reactivity using various DN data: -10percent and +2.7percent more or less 1.1percent (1 sigma) using respectively ENDF/B-VII.0 and JEFF3. A recommendation is finally proposed for improvement of DN data in international libraries.