Single-phase natural circulation loops are frequently used in the systems which are related to the production of renewable energies. Natural circulation systems have low driving head and consequently, low heat removal capability. Thus, minimizing the entropy generation is a critical matter for designing an optimized natural circulation system performance. Moreover, the instability behavior of these systems is one of their most important disadvantages. In the present study, the entropy generation and non-linear stability analysis for a uniform and non-uniform diameter loop under single-phase natural circulation are investigated. In this work, a constant heat flux condition in the heater and constant wall temperature condition in the cooler are assumed. The effect of various parameters such as loop dimensions and heater power on the single-phase natural circulation stability and entropy generation in the loop are analyzed. Finally, the stability of the system in the form of a stability map is discussed, and entropy generation within the loop is calculated. (C) 2014 Elsevier Ltd. All rights reserved.