The correct identification of the jet flavour (Flavour Tagging) is of fundamental importance for hadron collider experiments such as ATLAS and CMS at the Large Hadron Collider. In particular, it is an important tool for many physics analyses with heavy flavour jets in the final state (Higgs and top quark physics and SUSY searches). The new b-tagging algorithms developed for Run 2 improve the c-jet rejection with respect to Run 1 performance. A method to measure the efficiency to mistakenly tag a c-jet as a b-jet is presented. It uses a sample of c-jets produced in association with a W boson, with 2011 data collected by the ATLAS detector. The W+c-jet sample, with the W boson reconstructed via its decay into an electron and a neutrino and the c-jet identified via a soft muon stemming from a semileptonic c-hadron decay, allows to select a high-purity c-jet sample, exploiting the charge correlation between the electron and the muon.