The perturbation theory based on the transport calculation has been applied to study sensitivity of neutron multiplication factors (k eff 's) to neutron cross sections used for the reactivity analysis of UO 2 and MOX core physics experiments on light water reactors. The studied cross sections were neutron capture, fission and elastic scattering cross sections, and a number of fission neutrons, ν. The obtained sensitivities were multiplied to relative differences in the cross sections between JENDL-4.0 and JENDL-3.3 in order to estimate the reactivity effects. The results show that the increase in k eff , 0.3% k/kk , from JENDL-3.3 to JENDL-4.0 for the UO 2 core is mainly attributed to the decreases in the capture cross sections of 238 U. On the other hand, there are various contributions from the differences in the cross sections of U, Pu, and Am isotopes for the MOX cores. The major contributions to increase in k eff are decreases in the capture cross sections of 238 U, 238 Pu, 239 Pu, and those to decrease in k eff are decreases in ν of 239 Pu and increases in the capture cross sections of 241 Am. They compensate each other, and the difference in k eff between JENDL-3.3 and JENDL-4.0 is less than 0.1% k/kk and relatively small.