The Structural effect of IC plastic packages on the residual stress of silicon chips was discussed experimentally. The residual stress was measured by stress-sensing chips which authors have developed. Concerning the package structure, two different types of package were selected. One was the SOJ-type package, which is widely used for the surface mount technology, and the other was the COL-type package, which is newly developed for the large chip mounting technology. It is found that the residual stress of the silicon chip is varied more than three times by not only composition of package material but also package structure. The residual stress of the silicon chip encapsulated in the COL-type package is mainly determined by resin material. But the residual stress of the silicon chip encapsulated in the SOJ-type package is varied by combination of resin and lead frame matarial.