The present article tries to closely examine Gharar as one of the most important forbidden trades in Islamic canon and the foundational Islamic economic rules. In order to do so, at first, using library method and examining Shiite jurisprudential resources and the researches fulfilled in this area, it interprets Gharar, and then, adopting descriptive research method, utilizing mathematical argumentation, and simultaneously using game and contract theories, analyzes and studies this important economic rule. In order for this analysis to be performed, at first, investigating the literature on the kinds and defined features of Gharar and the theoretical foundations of game and contract theories, the article performs the theoretical modelling, and then examines the economic interactions and the enforcement of contracts in terms of social welfare and the well-being of the parties to the exchange. The results show that, on the basis of contract theory and game theory with developed structure as well, existence of Gharar reduces social well-being..