Present disclosure provides gate-all-around structure including a semiconductor fin having a top surface, a first nanowire over the top surface, a first space between the top surface and the first nanowire, an Nth nanowire and an (N+1)th nanowire over the first nanowire, and a second space between the Nth nanowire and the (N+1)th nanowire. The first space is greater than the second space. Present disclosure also provides a method for manufacturing the gate-all-around structure described herein.