The top quark was discovered in 1995 by the D0 and CDF experiments at the TeVatron. Due to its great mass, close to 172.5 GeV, the top quark is difficult to produce and some of its properties still need to be deeply studied. The Standard Model predicts two production modes which are the top quark pair production and the single top production. The top pair production is now well known unlike the single top production which exhibits lower production rates and higher background contamination. This thesis is devoted to the determination of the cross-section of the associated production of a top quark with a W boson in the dilepton channel using the ATLAS detector at the Large Hadron Collider (LHC). Two analyzes are presented in this document. The first analysis corresponds to the first measurement ever of the W+t process using the 2010 ATLAS dataset (35 pb-1 of data). This measurement is simple and robust and uses as much as possible data driven techniques for the various estimates of background contamination. A second analysis that uses the whole 7 TeV dataset collected during 2011 (4.7 fb-1) is also presented. This last study uses advanced discrimination techniques and refined statistical treatments in order to increase the global sensitivity of the measurement. The measured value of the W+t cross section is then used to constrain the top quark electroweak sector via a measurement of the CKM matrix element | V_tb |.
