Online petitions have been in the spotlight as an innovative mean for citizen participation in various countries over the past decade. The platforms’ great success promoted citizen participation in various aspects. However, inside the platform, petitions are losing their original purpose due to those containing inappropriate language, fabricated information and lacking tangible evidence for the petition. This also influences other users and eventually deteriorates into an environment where good petitions that could be used as a policy material cannot be generated. To solve this problem, this study aimed to identify design interventions that empower users’ deliberation and verify their effectiveness. Through an extensive literature review the significance and the potential effect of design interventions to improve deliberative capability was identified. Moreover, thorough content analyses were conducted to establish a framework for deliberative online petition. As an empirical study, causes for non-deliberative petitions were derived through user interview, along with a designer workshop conducted with 10 designers to define the needed design intervention. This work also implemented an online petition application prototype with six design interventions, revealing their efficacy through user surveys and interviews. The research showed that the design interventions deployed in the application augmented users’ deliberative mindset and capability, further synthesizing the major findings to construct design implications to guide designers and developers who want to build online petition platforms. The results of this study serve as a practical reference for developing a petition platform and are expected to contribute academically to the expansion of the scope of design intervention research into the context of citizen participation.