This article presents results of a theoretical and experimental study concerned with steady-state characteristics of a rectangular natural circulation loop with vertical heat transfer sections containing water near its density extreme. A one-dimensional model has been developed to simulate heat transfer behavior of the rectangular loop, with particular attention to unravel effect of density inversion of water near 4 degrees C. In parallel to the theoretical simulations, supporting experiments have been undertaken to obtain data to verify the model. Fairly good agreement was found between the predicted and measured temperature distribution along the loop. Influence of density inversion effect has been clearly demonstrated to be substantially significant and needs to be taken into account when the operating temperature of the natural circulation loop filled with water encompassing its density-extreme temperature.