In this paper, we studied a cooperative case of IT convergence between large firm and small firm through network externality effects. Specifically, we examined the strategic alliance background, the process, and the result of cooperation between Korea’s ICT company (KT) and the U.S software venture company (NEXENTA). KT, as a cloud service late-comer, chose an open flexible platform instead of general custom solution of expanding user network rapidly in order to catch up with the early mover. NEXENTA, a storage software late-comer, had been operating open software based perpetual license as a differentiated strategy from competitor’s package product, so was selected as a KT open cloud service platform partner. This alliance resulted in a great success. KT has become a global cloud service provider with explosive user expansion of uCloud since 2010. NEXENTA has also expanded its local customer pool to global customer pool. This study is particularly meaningful in the sense that it shows a successful international cooperation between a large firm and a small firm, while existing studies focused mostly on the domestic cooperation between firms.