A new one-dimensional thermal network modeling approach is proposed that can accurately predict transient/dynamic temperature distribution of passive cooling systems. The present model has applications in variety of electronic, power electronic, photonics, and telecom systems, especially where the system load fluctuates over time. The main components of a cooling system including: heat spreaders, heat pipes, and heat sinks as well as thermal boundary conditions such as natural convection and radiation heat transfer are analyzed, analytically modeled and presented in the form of resistance and capacitance (RC) network blocks. The present model is capable of predicting the transient/dynamic (and steady state) thermal behavior of cooling system with significantly less cost of modeling compared to conventional numerical simulations. Furthermore, the present method takes into account system “thermal inertia” and is capable of capturing thermal lags in various components. The model is presented in two forms: zero-dimensional and one-dimensional which are different in terms of complicacy. A custom-designed test-bed is also built and a comprehensive experimental study is conducted to validate the proposed model. The experimental results show great agreement, less than 4.5% relative difference in comparison with the modeling results.