Cyclic deformation and deformation substructure of CoTi single crystals were examined focusing on operative slip systems and cyclic hardening. Fatigue tests were performed in a tension/compression mode under a fixed amplitude of total strain (Δe = ±0.1-±0.3%) in air at room temperature. CoTi deformed by {110} slips at various strain amplitudes. At Δe= ±0.1% primary (110)[001] slip was dominant and the stress amplitude gradually increased with number of cycles until it reached a saturated value. As secondary slips were more activated with increasing Δe, cyclic hardening was more accelerated and the fatigue life was shortened. Highly piled-up dislocations composed of primary and secondary dislocations formed strong residual stress fields, resulting in crack initiation and propagation. A large number of dipoles and loops which also contributed to cyclic hardening were produced during the to-and-fro motion of screw dislocations.