A third classroom in colleges and universities is the expansion and extension of the traditional first classroom and second classroom. Based upon the development of network technology, a third classroom makes use of network resources and provides a better and more convenient chance for teachers and students to study and communicate with each other. This article first deals with the importance and necessity to construct a third classroom in colleges and universities. Secondly, it points out that the construction of a third classroom needs to improve teachers and students＇ abilities to apply network technology, and to get the support of hardware and software concerned. Last but not least, this article advocates that the construction of a third classroom can help realize the combination of a realistic classroom with a virtual classroom and the combination of in-class learning with the extracurricular learning.