This study aims to evaluate the performance of the global model of ionospheric slab thickness (GMIST) in terms of F2 layer critical frequency (foF2) estimation during geomagnetic disturbed conditions. Hourly values of foF2 as obtained from ionosonde stations located at equatorial, low- and mid-latitude regions are compared with the corresponding GMIST and IRI-STORM modeled values. For this purpose, the correlation coefficient, daily mean, root mean square error and improvement percentage are calculated at different regions and geomagnetic disturbance levels. The results show that GMIST is more accurate than IRI-STORM model in terms of foF2 estimation at low- and mid-latitude regions, while at equatorial areas GMIST is less accurate during geomagnetically disturbed and quiet conditions.