Abstract: We present a measurement of the W boson helicity fractions in ttbar events with the full dataset collected by the CMS detector in 2011 at a center-of-mass energy of sqrt(s)=7 TeV. This corresponds to an integrated luminosity of 4.6 fb-1. The analysis is based on the dilepton decay channel of top pair events, in which both W bosons decay leptonically. Only events containing electrons and/or muons in the final state are considered. We measure the fractions of left-handed, longitudinal, and right-handed W boson to be FL = 0.288+-0.035(stat)+-0.040(sys) , F0 = 0.698+-0.057(stat)+-0.063(sys) and FR = 0.014+-0.027(stat)+-0.042(sys). The results are consistent with the standard model predictions.