Abstract: Earthquake occurrence probabilities are estimated by applying the Semi-Markov Model. This model assumes that the successive earthquakes in the same structural discontinuity are not independent events, but they are influenced by the elapsed time interval between them. The North Anatolian Fault Zone between longitudes 26.00^O E and 42.00^O E is selected for this study. Seventy-one earthquakes having surface wave magnitudes M5.5 occurred in the region between 1902 and 1992 are considered. The occurrence probabilities were obtained by determining interval transition probabilities for region to region and magnitude to magnitude transitions of these earthquakes. The determination of joint probabilities demonstrates that earthquake occurrence can be investigated in the three dimensions of space, time and magnitude.