Probabilistic seismic hazard analysis (PSHA) is the most acceptable and reliable method for estimation of the earthquake hazard in different regions. The seismicity parameters including the mean seismic activity rate λ, b-value of Gutenberg–Richter, and the maximum possible earthquake magnitude Mmax are the most important inputs to PSHA. These influential inputs are contaminated with different source of uncertainties that can significantly affect the reliability of the seismic hazard assessment. In this study, the most dominant inputs affecting the assessment of the key seismicity parameters are evaluated in the province of Kohgiluyeh and Boyer-Ahmad province of Iran. The sensitivity analysis results show that the radius around the center of an area source could significantly affect the results in the region under study, in particular, the estimation of seismic activity. Moreover, the last (complete) part of the catalogue has a significant effect on the estimated value of seismic activity and the b-value. Furthermore, the most influential inputs for estimation of Mmax are the maximum observed earthquake and its uncertainty. Therefore, in regions with lack of precise data, probabilistic estimation of Mmax could be uncertain.