This paper is concerned with instability analysis of uncertain large-scale networks. First, we present an instability counterpart of the small gain-type robust stability condition for uncertain linear systems without network structure. Next, we extend the instability condition to that for a class of uncertain large-scale networks. Every node in the network has certain homogeneous dynamics and uncertain heterogeneous dynamics. Finally, an illustrative example is presented for instability analysis of an uncertain cyclic gene network model.