The paper explores the mediating effects of relationship fairness between relationship understanding and inter-firm trust. It takes the enterprise alliance within the region of Yangtze River Delta as the investigated object, utilizes a questionnaire survey method and obtains 230 research samples. Based on statistical analysis, the relationship understanding is proved to be pre-variable which may have impact on relationship fairness. Also, there are significant differences on the impacts in each dimension degree of relationship fairness caused by the component factors of relationship understanding. At the same time, the study also shows that relationship fairness has obvious mediating effect on relationship understanding and inter-firm trust while significant differences are existed in the paths of mediating effects in each dimension degree. Therein interactive fairness and distributive fairness are the most significant factors for mediating effects in every dimension degree of relationship fairness, and they also have complete or partial mediating effects on the factors of relationship understanding and inter-firm trust. However, there is no prominent mediating effect caused by procedural fairness. Accordingly, some suggestions are proposed for China's alliance management.