Kinesin motors power many motile processes by converting ATP energy into unidirectional motion along microtubules. The force-generating and enzymatic properties of conventional kinesin have been extensively studied; however, the structural basis of movement is unknown. Here we have detected and visualized a large conformational change of a ~15-amino-acid region (the neck linker) in kinesin using electron paramagnetic resonance, fluorescence resonance energy transfer, pre-steady state kinetics and cryo-electron microscopy. This region becomes immobilized and extended towards the microtubule `plus' end when kinesin binds microtubules and ATP, and reverts to a more mobile conformation when γ-phosphate is released after nucleotide hydrolysis. This conformational change explains both the direction of kinesin motion and processive movement by the kinesin dimer.