This study analyzed the factors influencing the effectiveness of inter-organizational relationship (IOR) through relationship marketing of travel companies with their partners based on data collection with 256 responses who were working for travel companies in Vietnam. The study used a quantitative approach as factor analysis, multiple regressions and path analysis. The results indicated that inter-organizational relationship effectiveness was directly and indirectly affected by coordination, trust, frequency of interaction, commitment, communication, organizational compatibility, and relationship marketing. Surprisingly, formalization did not significantly affect the effectiveness of IOR and relationship marketing. This study is an essential reference for managers to make decisions and to enhance the effectiveness of their collaborative partnerships.