Factors affecting the time response function of a fast lifetime spectrometer were identified and investigated. For this purpose, a fast/slow gamma-gamma coincidence system was constructed using fast types of scintillators and photomultiplier tubes. The measured response functions obtained at various conditions were analyzed using a computer program code "RESOLUTION" The following factors were considered: High voltage power supply of the PMT. the time pick-off technique, the external shaping delay time, walk, resolving lime of the coincidence unit, the dynamic range, the photon energy, the ²²Na energy windows and the source-detectors geometry. The results indicate that factors affecting the timing spectrometer can be . classified into two groups; the first, those have a significant effect on the time response function, namely; the time pick-off technique, the resolving time, the dynamic range and the photon energy. A considerable improvement in the shape of the time response function can be achieved when these factors are considered. The second group, those which have only small effect on resolution, namely; :the high voltage supplied to the detector, shaping delay time, walk, 22Na energy windows and source-detector geometry. A significant effect on the shape of time response function was observed when using the CF and SRT timing techniques instead of LE technique. A nearly negligible effect of this factor on the coincidence rate was detected. The resolving time of the coincidence unit have a significant effect on the time resolution and coincidence rates of the spectrometer. The coincidence rate for a certain energy window is nearly independent on resolving times longer than 40 ns. The dynamic range and photon energy have an important role in the time response function of the fast lifetime spectrometers. At narrow dynamic ranges, a considerable improvement in resolution is achieved and a low coincidence rate was associated with it. The resolution deteriorates as the photon energy decreases and vice versa. For certain energy window, the resolution is improved as the difference in the energy of photons in coincidence is decreased. The ²²Na. energy widow setting, significantly affect the time resolution and coincidence rate. The time resolution is nearly independent on small changes in the source-detector geometry. However, these changes causes a large shift in peak position and therefore accurate time - zero determination requires the use of symmetrical source-detector geometry. The present spectrometer was tested by measuring the positron lifetime in aluminum sample prior to and after a low deformation level. The results demonstrate that the spectrometer is sensitive to ultrashort time intervals.