The steady-state characteristics of natural circulation loop (NCL) were investigated by a homogenous model with SINDA/FLUINT. Under different condition of heat loads, cooling temperature, friction of the loop and driving force etc., the NCL can be operated in different operation modes, such as all two-phase, all liquid, two-phase-liquid, or gas-two-phase-liquid etc. A pair of non-dimensional parameters, namely eta(p) and eta(l) is proposed to characterize the operation modes of the NCL. eta(p) stands for the relative size of the driving force, while eta(l) stands for the non-dimensional length of condensation, which describes the capability of the condenser. Three structures of NCL with water and totally over 1000 cases were run. The result was summarized in the eta(p)-eta(l), maps. They present the boundaries that all of four different operation modes in the map, which means the two non-dimensional parameters eta(p) and eta(l) can be used for scaling the operation modes. (C) 2015 Elsevier Ltd. All rights reserved.