Two-phase flow is widely used in the cooling system of electronic equipment, where the heat source is localized. However, two-phase flow may lead to flow instabilities, which have adverse effects on the system such as heat transfer deterioration and vibration. In this study, the effects of related variables on Ledinegg instability in single and parallel channels under localized heat source are investigated. Both models of single and parallel channels have been verified, and the maximum relative error is within ±5%. Based on these two models, the results indicate that the high heating power and low inlet subcooling promote Ledinegg instability, whereas Ledinegg instability is independent of the channel inclination. The average negative slope of continuous heat source, −0.09557, is smaller than that of the heat source close to inlet, −0.07408, which shows the improvement of localized heat source on Ledinegg instability. For the parallel system, when the flow excursion occurs, the total mass flow rate, which is 0.028 kg/s, is equal to two times of the mass flow rate of maximum value in branch pressure curve at which pressure is equal to 0.778 MPa, if two branches are identical. The current work provides qualitative conclusions on how Ledinegg instability will be affected with the change of relevant variables.