The objective of this study is to assist in understanding and predicting the thermal operating behavior of electrical devices in vehicle Electrical Distribution Box (EDB), and to provide a thermal design model which can be used to aid new EDB design. In this work, the temperature characteristics of one of the electric devices, Micro ISO Relay, in EDBs, were studied in detail through the combination of experiments and finite element analysis from the individual device level to the assembled EDB level. This study reveals the relationship of the temperature profile in EDBs with a wide range of variables, such as current loads, device location, content and operating environment. A thermal design model including the current loads and geometric arrangement of the electrical devices in EDBs was parametrically developed. A good correlation between the experiment and the model's prediction was observed.