This study aimed to identify and compare the impact of factors in Cobb-Douglas production function on the value added of Vietnam Airlines and VietJet Air. Analyses of Vietnam Airlines data for the period of 2007-2016 and VietJet Air for the period of 2012-2016 show that the contribution coefficient of total assets and labor costs to the value added of VietJet Air is higher than Vietnam Airlines and the growth of Vietnam Airlines' value added depends on total assets, labor costs as well as total factor productivity. The research results also give that Vietnam Airlines no longer presents economies of scale advantage. However total factor productivity still plays the key role in Vietnam Airlines' growth. In contrast, VietJet Air is in the early stages of development therefore it presents economies of scale advantage and it is developing strongly. These findings is the basic to suggest policies for Vietnam Airlines and VietJet Air improving their competitiveness and business efficiency.