Fatigue crack closure and crack growth behaviour in Ti-2.5 wt% Cu alloy with two equiaxed and two lamellar microstructures have been investigated by constant-load amplitude tests. Plasticity-induced crack closure and roughness-induced crack closure have been characterized separately by experimental methods. A change in closure mechanism from plasticity-induced crack closure at high Delta K values (region of high stress intensity ranges) to roughness-induced crack closure at low Delta K values occurs in a solution-annealed equiaxed microstructure, while plasticity-induced crack closure is the operative closure mechanism in an over-aged equiaxed microstructure over the whole range of Delta K and roughness-induced crack closure occurs in two lamellar microstructures. The crack closing stress intensity factor for plasticity-induced crack closure increases continuously with increasing maximum stress intensity. The crack closing stress intensity factor for rough ness-induced crack closure increases with increasing maximum stress intensity at low Delta K, and remains constant at high Delta K. Crack closure and crack path deflection have a significant influence on the crack growth rates.