In the present work, the heat transfer and fluid flow characteristics of a single-phase natural circulation loop (SPNCL) with both the heating and the cooling ends are theoretically and experimentally studied. Distilled water is chosen as the working fluid circulating in a rectangular loop that the top and bottom sides are respectively cooled and heated by 245 mm tube-in-tube heat exchangers. The height and diameter of the mini-loop are 250 mm and 4 mm, respectively. Both analytical and experimental results are obtained by varying the heating fluid temperatures from 30 degrees C to 60 degrees C but fixing cooling fluid temperature of 10 degrees C. Based on the experimental data, a Nu similar to Re correlation is obtained and applied into the one-dimensional mathematical model. A good agreement between the experimental and theoretical results with a new correlation can be observed. Experimental results show that stable flow can be reached for the cases with different T-h. The start-up time of natural circulation from quiescent state shortens with the increase of T-h. The Reynolds number and heat transfer rate at steady state are proportional to the heating fluid temperature T-h. Based on the proposed mathematical model, an optimal ratio of the heater length to the loop height can be reached when the total length and diameter of the mini-loop keep constants. (C) 2018 Elsevier Ltd. All rights reserved.