Photovoltaic power generation systems are one of the main renewable power sources, and quasi-Z-source inverters are becoming powerful devices to integrate these systems in AC grids. However, stability issues due to the damping behaviour of converters must be considered. There are several studies in this direction but instability concerns are not completely solved yet. This paper contributes with a procedure for the stability assessment of photovoltaic power generation systems with quasiZ-source inverters in the frequency domain. The study is based on the small-signal averaged model of the system expressed in the s-domain and the stability criterion derived from the frequency characteristics of the state-space matrix. The influence of the photovoltaic power generation system operating point on stability is studied by the proposed procedure. Eigenvalue analysis and PSCAD/EMTDC simulations are also performed to validate the obtained results.