Measurement of top quark properties at CMS

Measurements of top quark properties in top quark decays are presented, using data collected by the CMS experiment during the years 2011 and 2012. The polarization of W bosons in top quark decays is measured. The W boson helicity fractions and angular asymmetries are extracted, and limits on anomalous contributions to the Wtb vertex are determined. Furthermore, searches for flavor changing neutral currents in top quark decays are presented. The flavor contents in top quark pair events are measured using the fraction of top quarks decaying into a W boson and a b quark relative to all top quark decays, $R=BR({\rm t} \to {\rm Wb})/BR({\rm t} \to {\rm Wq})$, and the result is used to determine the CKM matrix element $V_{\rm tb}$ as well as the width of the top quark resonance.