The characteristic performance of a photon counting detector for X-ray fluorescence (XRF) imaging of gold nanoparticles (GNPs) is investigated. The investigations are first performed in three aspects: X-ray photon energy (keV) to pulse height (mV) conversion, noise floor determination, and linear detection ranges. Then, theoretical models are applied to evaluate the detection efficiency of X-ray photons with respect to an increased incident photon rate. Last, through exciting 100% pure GNPs by a conventional X-ray tube operated at a voltage of 110kVp, we acquire XRF spectrum in the threshold mode, based on which multi-energy thresholds are selected for XRF imaging of GNPs with low concentrations. Preliminary XRF imaging results of GNPs obtained in the imaging mode are presented and analyzed. This investigation study is essential to the development of fast and accurate XRF imaging of GNPs as well as other high atomic (Z) imaging contrast agents absorbed in cancerous cells.