In recent years, the importance of long-term relationship formation in the transactional exchange activities between suppliers and organizational buyers in industrial markets has been greatly highlighted. Such buyer-supplier relationships are determined by formal or informal contracts, but it is virtually impossible for each transacting party to conclude contracts that consider all possible future contingencies, and when the business environment changes, either the buyer or the supplier will seek to modify or alter existing transaction contracts to adapt to environmental changes. Under these circumstances, this study empirically analyzed how flexibility in the industrial supplier's transactional relationship activities with organizational buyers affects the buyer's relational transaction outcomes (satisfaction, relationship quality, trust, commitment) with the supplier, and also analyzed the antecedent factors influencing flexibility (market uncertainty, relationship-specific assets, opportunistic behavior, ambivalence, long-term orientation) from the perspectives of transaction cost and contracting theory. Finally, this study also analyzed the moderating roles of planning level and conflict management level in the relationship between flexibility and relational transaction outcomes. The empirical analysis of this study was conducted through a survey, and the questionnaire was administered to organizational buyers purchasing parts and raw materials in industrial markets. Respondents were asked to select one important supplier relationship, and organizational buyers with transaction periods of less than two years were excluded. A total of 224 questionnaires were used in the empirical analysis employing structural equation modeling. The analysis results showed that the goodness-of-fit for the structural model was sufficient, and as hypothesized in the study, opportunistic behavior, relationship-specific investment, ambivalence, and long-term orientation were found to have statistically significant effects on supplier flexibility. Furthermore, flexibility was found to have significant effects on relational transaction factors (relationship satisfaction, relationship quality, trust, and relationship commitment). Additionally, regarding the moderating role that conflict management was hypothesized to play in the relationship between flexibility and relationship satisfaction, relationship quality, trust, and relationship commitment, meaningful differences were found through between-group path coefficient analysis.