A quick and easy way of determining activation cross sections by the analysis of γ-ray spectra is described. To determine cross sections by this method, it is not necessary to measure the neutron flux density, to undertake any chemical separation process, nor to use enriched target substances. To ascertain the validity of the method, the activation cross section of 102Ru(n,γ)102Ru reaction was obtained, resulting in the value of 1.37±0.132 barns, which compares well with published data of 1.44±0.16 barns, determined by another method.