In this study the properties of the aftershock sequences in the first five months after the 17 August 1999 Izmit and 12 November 1999 Duzce earthquakes are investigated. For this purpose, the b-value of frequency-magnitude distribution of earthquakes and temporal decay rate of aftershocks as described by the p-value in the modified Omori law are calculated. The data taken from the website of the KOERI consist of 1841 events for the Izmit and 913 events for the Duzce activity, respectively. The b-value for the Izmit sequence is found as 1.10 +/- 0.03 with Mc = 2.6 and b = 1.16 +/- 0.05 for the Duzce sequence with Mc = 2.8. Both b-values are close to 1 and typical for aftershock sequences. The p-values are calculated as 0.86 +/- 0.05 for the Izmit and p = 1.34 +/- 0.09 for Duzce aftershocks, respectively. Small p-value found in Izmit sequence may be a result of the background seismicity of the aftershock region. Because of the lower p-value for the Izmit activity than that of the Duzce sequence, the aftershocks show a relatively slow decay rate in the Izmit earthquake. Considerable spatial variability is found, b-values ranging from 0.8 to 1.6, and p-values ranging from 0.4 to 1.4. The spatial variations of b and p-values of an aftershock sequence may associate with the rupture mechanism and geological structure of an earthquake area. A good agreement between the slip and b-value for the Duzce sequence are found whereas we could not observe a linear relation between these parameters for the Izmit sequence. After the lzmit earthquake, the larger b-values correlate with the regions where the lower stress changes occurred whereas the lower b-values are in the areas with the larger stress variations. Also, the lower and higher b-values are related to the higher and lower P-wave velocity in general. Regarding the geological aspect, the larger b and p-values for the two sequences are related to Holocene alluvium structures which have low velocity. On the other hand, after the both earthquakes, the larger p-values correlate well with the regions on which the maximum slip is located and the lower p-values are related to the lower slip regions. However, the material properties seem more effective than the stress changes and slip distributions in the b-value variations and the slip distribution is the most significant factor on p-values in the both sequences.