Ground-motion attenuation relationships using the 1999 Kocaeli earthquake data were developed for the Northwest Anatolia region. This region is seismically active due to its location on Northwest Anatolia Fault Zone and was affected by the 1999 Kocaeli and Duzce earthquakes. Properties of the investigated stations and strong ground-motion data were taken from the Strong Ground Motion Database of Turkey (2017) (TR-NSMN) and Pacific Earthquake Engineering Research Center-Enhancement of Next Generation Attenuation Relationships for Western US (PEER-NGA-West2) database. SeismoSignal software was used in the evaluation of the acceleration records measured in the stations. A generated database for this study contains 369 mainshock and aftershock records, which occurred in the region of 39.39 to 41.03 North (N)/26.04 to 31.73 East (E) coordinates between the years of 1999 (Kocaeli earthquake) and 2006. In this research, peak ground acceleration is greater than 1 gal, and moment magnitude (M-W) is greater than 4.0 and Joyner-Boore distance (R-JB) is 1-344 km. These records were taken from 76 stations located in the investigation area. In addition to these data, 33 mainshock records worldwide were used for recovery of regression coefficients. Therefore, total of 402 data were used in this research. Attenuation relationships obtained from different types of ground were derived from the model generated by Boore et al. (Seismol Res Lett 68(1): 128-153, 1997) for shallow earthquakes in North America. In this study, attenuation relation equations were developed by applying nonlinear regression analysis, with Statistical Package for the Social Sciences (SPSS) Statistics 20.0 software for B-C and D class soil according to the National Earthquake Hazards Reduction Program (NEHRP) classification system.