Experiments on the reactivity worth of beryllium metal were performed using the Kyoto University Critical Assembly, and they were analyzed to examine the validity of the computational method to treat (n,2n) reactions in calculations. The experimental results demonstrated that beryllium metal has positive reactivity worth compared with graphite. In the analysis, (n,2n) reactions were treated as modifying scattering cross sections in a transport calculation, whereas both scattering and absorption cross sections should be modified in a diffusion calculation. The results of calculations for the reactivity worth of beryllium agreed with experimental data within a few percent in the calculated-to-experimental ratio. Calculated results indicated that (n,2n) reactions of beryllium contribute by ∼85% to the positive reactivity worth compared with graphite in these experiments at a thermal reactor. Moreover, through the improved neutron and gamma-ray coupled calculation, the effect of (γ,n) reactions of beryllium on reactivity was estimated. It was found that (γ,n) reactions of beryllium can be negligible so far as this reactivity worth is concerned.