Previous studies showed the effects of psychological variables such as depression and loneliness on game addiction. Likewise, the effect of aggression on game addiction has also been reported. However, little research has empirically proved the role of aggression between the psychological variables and game addiction. In addition, in the context of game addiction, few studies have investigated their relationships in an integrated model using sub-factors of each construct. With survey data of 789 subjects, focused on the psychosocial variables (e.g., aggression, depression, and loneliness), the current study analyzed a path model to examine 1) the effects of loneliness and depression on aggression, 2) the psychosocial variables' effect on game addiction, and 3) mediation role of aggression between psychological variables (e.g., depression and loneliness) and game addiction. Results showed the important roles of aggression in the model. Aggression was significantly affected by both loneliness and depression, and in turn, aggression was the strongest determinant on game addiction among the three variables. Notably, aggression played a mediation role between depression and game addiction. Results and implications were discussed.