One of the main challenges associated with two-phase natural circulation loops (NCLs) is the thermal-hydraulic instability. The prevailing system conditions can take NCLs into single-phase or two-phase flow conditions. The starting of single-phase NC and subsequent transition into two-phase is very important for system safety and the models developed should be capable of capturing these phenomena. The phenomena involved in the start-up of two-phase NCLs are depending on various parameters. With this objective, the authors have developed single-phase NCL model in their previous work. In the present study, the model is further extended to two-phase NCLs using homogeneous equilibrium model to investigate the transients of two-phase NCLs. The governing equations are solved based on finite volume method (FVM). The influence of wall conduction is considered by including wall conduction model. The model has been validated against three experimental NCLs. Subsequently, the validated model is employed to investigate the start-up thermal-hydraulics of two-phase NCL i.e. starting from single-phase, transition and then to two-phase flow for the chosen NCL configuration. Further, the influence of various parameters like input power, loop height, loop diameter and wall thickness on the start-up dynamics of two-phase NCLs are studied and discussed in this paper.