With the app1ication of heat-transfer theory an equivalent thermal network (electrothermal analog circuit)is. set up by electrothermal analogy which is for FECL100K under the circumstance of forced air cooling. The mathematic model of heat-transfer for the network is also given. It provides a foundation for computer aided thermal design. By applying the equivalent thermal network and its model the reasonableness of placement form of this package in electronic equipment is expounded.