A high temperature and pressure dual-channel facility has been designed and developed at Multiphase Reactors Engineering and Applications Laboratory (mReal) at Missouri University of Science and Technology (Missouri S&T) to investigate the thermal hydraulics of natural circulation under the loss of flow accident scenario (LOFA) in the core of a prismatic modular reactor (PMR). An advanced heat transfer measurement technique consisting of a series of heat flux foil sensors and thermocouples is adapted and implemented along the coolant flow channels (riser and downcomer channels). This study presents the effect of nonuniform isoflux heating (1.432 kW/m2 to 3.152 kW/m2, in form of stepwise-reducing and stepwise-increasing) on natural circulation heat transfer using air as a coolant at 413.7 kPa. The results showed a reversal in the direction of heat transfer and reduction in temperature fields as well as observed within the riser flow channel close to the exit (Z/L = 0.773) due to the end effect. A significant reduction of the average field temperatures along the riser channel is observed to be 24.5% and 22.4% for inner surface and air centerline temperatures, respectively for stepwise-reducing. Moreover, for stepwise-increasing, a significant reduction of the average riser channel temperature fields is observed with decreasing stepwise-increasing heating by 25.6% and 23.1% for the inner surface and air centerline temperatures, respectively. The average values of the heat transfer coefficient(hL) and the average Nusselt number (NuL) along the riser channel were decreased with reducing the heating by 38% and 19% for stepwise-reducing heating, respectively, and by 32% and 13% for the stepwise-increasing heating, respectively.