A one-dimensional theoretical model has been used to analyze the steady state and stability performance of a single-phase, two-phase, and supercritical natural circulation in a uniform diameter rectangular loop. Parametric influences of diameter, inlet temperature, and system pressure on the steady state and stability performance have been studied. In the single-phase liquid filled region, the flow rate is found to increase monotonically with power. On the other hand, the flow rate in two-phase natural circulation systems is found to initially increase, reach a peak, and then decrease with power. For the supercritical region also, the steady state behavior is found to be similar to that of the two-phase region. However, if the heater inlet temperature is beyond the pseudo critical value, then the performance is similar to single-phase loops. Also, the supercritical natural circulation flow rate decreases drastically during this condition. With an increase in loop diameter, the flow rate is found to enhance for all the three regions of operation. Pressure has a significant influence on the flow rate in the two-phase region, marginal effect in the supercritical region, and practically no effect in the single-phase region. With the increase in loop diameter, operation in the single-phase and supercritical regions is found to destabilize, whereas the two-phase loops are found to stabilize. Again, pressure has a significant influence on stability in the two-phase region.