In the theory and practices of international relationship marketing, relationship quality (RQ) has been paid more and more attention to in the research of enterprises' multi-national business. This paper begins with the analysis of the characteristics of relation and relationship quality, and explains the effect of RQ on the international purchasing alliance decision by a causal analysis model set up. In this model, by theoretical analysis, develop four factors to describe RQ and make an empirical analysis with the answers of 151 purchasing managers in the questionnaire. The result shows that RQ has been the critical factor of international transaction decision.