A study on the start-up phases of a capillary pumped loop for terrestrial application (CPLTA) is proposed in this paper. Experimental analysis and numerical modeling, using a one-dimensional spatial discretization model, based on thermohydraulic equations and solved by nodal network/electrical analogy, are presented to study the thermal and hydraulic behavior of the loop for methanol and n -pentane as working fluids, during start-up transient phases. The experimental observations are backed up by the numerical model to help the transient and steady analysis of this kind of loop. The precise numerical study allows to have a better understanding of the complicated phenomena happening during the start-up and to have a global view of the behavior of the capillary pumped loop for integrated power (CPLIP) during these phases. In this study, it will be also shown the influence of vapor line solid walls thermal inertia and its impact on the dynamic behavior and on the success of the start-up of the loop.