Observations of the geomagnetic total intensity have been carried out from time to time in a possible seismic gap region in the western part of the North Anatolian Fault Zone (NAFZ), in an attempt to trace faultlines through magnetic characteristics and also to relate the characteristics to fault activity. Here we report the results of profile measurements made across the Iznik-Mekece fault as well as our interpretation. The anomalies which we found near Iznik City are similar with the ones that have been found along the eastern part of the lznik-Mekece fault and hence they could be interpreted in terms of a dike-like structure in the andesite zone adjacent to the fault. In this area the northern edge of the dike-like structure turns out to correspond to the faultline, providing a piece of evidence for the location of faultline. In addition, the dip angle of the dike-like structure provides information on the dip-slip constituent of the primarily right-lateral Iznik-Mekece fault.