This study aims to elicit smallholder farmers’ preferences for the design of certification standards. We conducted a randomized conjoint analysis (RCA) with 745 green smallholder tea farmers in Vietnam. This method enables us to estimate the causal impacts of design attributes on farmers’ participation, which is a knowledge gap addressed by few empirical studies. Our results revealed that compared to the price premium, certification fee has a much stronger impact on farmers’ participation. Further, we find the potential to incorporate the application of organic fertilizer, a widely recommended sustainable input, into the certification schemes. Based on our causal findings, we suggest policies for improving the coverage of certification standards.