An effective thermal verification method for spacecrafts is introduced, in which their partial model or structural segment can be used in the test. The essence of this concept is to evaluate thermal design of the spacecraft by integrating a thermal vacuum test into thermal analyses system as a data generating subroutine. The measured temperature data are used for modification of the thermal mathematical model, which in turn requires to generate new test data for further refining of the model. Practically, the verification process is a kind of numerical simulation using the in-situ temperature measurements obtained from the thermal vacuum test run simultaneously with the simulation analysis. This direct coupling or “hybrid simulation” process is terminated when the thermal mathematical model is finalized to determine the spacecraft hardware design. This new method is capable of evaluating the thermal design of a large scale spacecraft in time and cost effectiveness with reasonable preciseness.