Positron annihilation lifetime spectra and Doppler broadening linesliapes of positron annihilation radiation energy were measured for the tension-compression fatigued Ni specimens under constant strain amplitude and for the cold rolled ones which were used for comparison. A few of the formers were selected for TEM observation. A lifetime component that can be associated with e+ lifetime in vacancy clusters developed during fatigue process was resolved by using a multi-exponential fitting method. It is 209, 255 and >300 ps on average in diferent ranges of fatigue respectively. The change of its relative intensity seems associated with increase of the concentration of the small vacancy clusters at early stage and growth of its dimension afterwards.Hence, this work provides a experimental evidence for the formation and clustering of vacancies as well as the development of a great number of jogged dislocations in fatigued Ni. The concentration of these defects were estimated by using a simple three state e+ trapping model. This work also provides a illustration of the utility of multi-exponential fitting method to complex lifetime spectra and to the above mentioned trapping model.