Based on the concept that relational contracts complete a formal contract in the absence of a legal enforcement mechanism, this study assessed the coexistence of relational and formal contracts to reduce side selling in sesame contract farming. Thus, trust and communication are used as relational contract elements, and the need for a formal contract was assessed based on transaction cost attributes (asset specificity and behavioral uncertainty). A structural equation modeling was used to analyze the relationship between the attributes of relational contracts and transaction cost economics and their direct and indirect effect on reducing side selling. The result indicates trust has a positive significant direct effect on reducing side selling with an effect coefficient of 0.248. Based on the result, while asset specificity, satisfaction, communication, and commitment have a total significant positive effect on reducing side selling, uncertainty encouraged side selling. Even if it is appealing for farmers to side sell when the market price is higher, the cost of losing secured market outlet and long term buyer exceed the short term benefit. Therefore, both farms and the firm need to foster trust to have a sustainable relationship. Alongside trust, it is better to execute contract legislation and enforcement mechanisms to circumvent uncertainty.