The electrochemical lithiation/delithiation processes of InAs nanowires (NWs) are studied by in-situ transmission electron microscopy. Our results indicate that InAs NWs have a fast lithiation speed of 275nm/s and a high lithium ion (Li-ion) diffusion coefficient of 2.49×10−8 cm2/s at room temperature. Upon lithiation, the Li-ion insertion firstly results in severe lattice distortions of InAs NWs, and the formation of Li3As and Li x In through the conversion and alloying processes take place on further lithiation. A small volume expansion of 157% is observed in full lithiation and is attributed to the naturally formed surface oxide layer. During the delithiation process, volume contraction and the dealloying of Li x In take place. Induced by the alloying and dealloying of Li x In, the dark and bright strips along the basal plane of InAs NWs appear and disappear alternately during the lithiation–delithiation cycling. Our results provide important insights into the lithiation/delithiation mechanism of III–V group nanomaterials and are envisaged to be helpful for designing lithium ion battery anode materials with fast lithiation speed, small volume expansion and reversible lithiation/delithiation processes.