An imaging detector as a base unit for a gamma camera for dual-modality (positron and single-photon) molecular imaging is developed in this study. The imaging detector is constructed using scintillation material lutetium yttrium oxyorthosilicate (LYSO) and a position-sensitive photo-multiplier tube. Performance indices for dual-modality imaging applications are measured and the detector’s feasibility is evaluated. Since LYSO exhibits spontaneous radiation，the practicality of the unit for single-photon imaging is evaluated. The results show that mean pixel widths of 0.6 and 0.8 mm and energy resolutions of 10% and 15% are achieved for 511 keV and 122 keV gammas，respectively，with a crystal pixel size of 1.5 mm. This pixel size is shown to be an engineering limit for meeting the requirement of the two imaging modalities. The background count rate from spontaneous radiation of 69 counts per second (cps) is less than 1/10 the size of the recorded signals in pinhole-collimated imaging. It is thus concluded that the proposed LYSO-based detector unit can be applied to dual-modality molecular imaging for clinical use with outstanding performance.