Abstract: In this paper, by using the free valence indices and Dewar's activity indices as the structure parameters, these parameters for 19 PAH and the second-order rate constants of the Diels-Alder reaction are regressed linearly., The correlations have been satisfactory. The position for the Diels-Alder reaction of PAH can be deduced from the structure parameter. The reactoin-position of 20 PAH are determined by means of the free valence indices and Dewar's activity indices, the results are consistent with the position determined by other structure parameters and experimental facts. We use resonance structure counts to deterrine the position not only with excellent correlation from HMO parameters, but also with a simple method for the calculation.