The issue of decision-making in an international negotiation is always one of the concerns of statesmen, officials and diplomats, because the wrong decision of a diplomat leads to failure in the negotiation. Game theory, which has attracted the attention of experts in various sciences, including political science, tries to make the best decisions in the decision-making and policy-making cycle on major issues to make decisions more rational and useful. In particular, the negotiations that are taking place today in the field of international relations can be examined on the basis of game theory and an agreement can be reached. In this research, we simulate the general process of a negotiation between the two countries with the help of game theory. Simply put, we simulate the actual process of a negotiation using a set of mathematical methods, models, and tools. In this regard, we classify negotiation into four stages: preparation for negotiation, exchange of proposals, reaching an agreement, and ending the negotiation. We model the preparation phase for the negotiation using finite game models and provide algebraic methods for calculating dominant performance and Nash equilibrium. According to the results of the first step, we model the exchange of suggestions with the help of dynamic game tree model. In the third stage, by solving tree models with the backward induction, we obtain the optimal strategies of the players. At the end of the negotiation, we will examine the conditions for reaching an agreement. Finally, a decision-making process in a negotiation is presented.