Résumé : The first measurement of the cross section ratio σttbb/σttjj is presented using a data sample corresponding to an integrated luminosity of 19.6 fb-1 collected in pp collisions at √s = 8 TeV with the CMS detector at the LHC. Events with two leptons (e or μ) and four reconstructed jets, including two identified as b quark jets, in the final state are selected. The ratio is determined for a minimum jet transverse momentum pT of both 20 and 40 GeV/c. The measured ratio is 0.022±0.003(stat)±0.005(syst) for pT>20GeV/c. The absolute cross sections σttbb and σttjj are also measured. The measured ratio for pT>40GeV/c is compatible with a theoretical quantum chromodynamics calculation at next-to-leading order.