The paper presents the measurement of the spectrum averaged cross section of 63Cu(n,2n)62Cu reaction in 252Cf spontaneous fission neutron spectrum. The spectrum averaged cross section in 252Cf spectrum was chosen as a validation tool since 252Cf is the only standard neutron field and 62Cu isotope is not easy to measure by gamma spectroscopy since line of interest is an annihilation peak, which is also produced by 64Cu isotope. Fortunately, contributions to peak from these isotopes can be distinguished due to the very different half-lives. Spectrum averaged cross section was inferred from the experimental reaction rate. The spectrum averaged cross section in 252Cf spontaneous fission neutron field for the 63Cu(n,2n)62Cu reaction was determined as equal to (0.1763 ± 0.0077 mb). This value agrees with value (0.183 ± 0.007) mb within uncertainty presented by Mannhart. However, it differs by 12.7% from IRDFF-II value which is equal to (0.19874 ± 8.954E-3) mb. Furthermore, reasonable agreement is not achieved with ENDF/B-VIII, JEFF-3.3, CENDL-3.1, ROSFOND-2010 nor JENDL-4 nuclear data libraries.