Free-to-play business model, which first commercialized in Korea, now becomes crucial sales drivers in the game industry, but the theoretical background is not well known that most of free-to-play content business models are developed based on the guts and trial-and-errors. In this study, we verified that the price discrimination theory in economics is the backbone of the free-to-play business model, and we also derived the three boundary conditions that should be satisfied in the business model design. We reviewed the three boundary conditions of free-to-play business model using case studies of previous games, and showed how the boundary conditions should work in the actual business. Through case studies, we tried to suggest the theoretical basis of free-to-play business model design, and sales enhancing techniques in free-to-play business.