We made a prototype gamma camera system with CdZnTe semiconductor detectors that work at room temperature. The effective field of view of this detector is 195 x 156 mm2 with a pixel size of 2.46 x 2.46 mm2. We evaluated the performance of the detector in terms of the energy resolution, intrinsic spatial resolution, system spatial resolution and system sensitivity. The performance of the detector was evaluated with methods based on the NEMA standard (NU1). The results showed that the mean energy resolution was 5.55 % FWHM for 99mTc, the intrinsic spatial resolution 2.46 mm FWHM, the system spatial resolutions 7.1 mm FWHM (without scatterer) and 7.4 mm FWHM (with scatterer), and the system sensitivity 46.3 cps/MBq. The performance of the system was also evaluated with some SPECT phantoms and the feasibility of the detector was confirmed with the SPECT images.