As it is known that spectrum is one of the most scarce and expensive sources, the urgent need to use the spectrum more effectively becomes necessary for the future wireless communication. Depending on the concept of cognitive radio, available bands in the licensed spectrum can be estimated, and allocated for unlicensed users. Therefore, accurate detection of spectrum occupancy is very important to deploy cognitive radio successfully. This study aims to determine the radio spectrum usage in Samsun. To reach this goal, extensive measurements were performed at 115 locations in GSM900 band using RF Explorer 6G Combo. Based on the measurement results, spectrum occupancy percentages were calculated for eight different thresholds from -40 dBm to -75 dBm. The measurement results, and occupancy calculations show that the spectral occupancy in Samsun is about 8.5% for -40 dBm while 82% for -75 dBm. It can be also concluded from the results that, GSM900 band is almost completely occupied for -75 dBm. According to measurement results what percentage the GSM900 spectrum band is occupied can be evaluated.