Wolbachia and Cardinium are both maternally inherited intracellular bacteria that infect a wide range of arthropods and are associated with various reproductive abnormalities in their hosts. Cytoplasmic incompatibility (CI) is the most common effect associated with the infection of endosymbionts. The CI results in a failure of karyogamy when infected males mate with uninfected females. Although infection by both Wolbachia and Cardinium is fairly common, little is known about their interactions and each symbiont's contribution to the phenotype of doubly-infected hosts. In this study we measured the strength of CI induced by Wolbachia, Cardinium, or both in the Hainan population of Sogatella furcifera (Horváth) by performing all possible crossing combinations, and show that Cardinium induced strong CI, Cardinium and Wolbachia together caused partial CI, and Wolbachia did not induce CI. We found that doubly-infected females could slightly rescue CI induced by Cardinium-infected males, Cardinium-infected females could also weakly rescue CI induced by Cardinium-infected males. The investigation of the fitness costs, benefits, or both of infection by each bacterium in planthopper showed that the infection with Cardinium alone could shorten the developmental time of nymphs in the Hainan population. Based on these results, we estimated that Cardinium may be the leading factor of CI in S. furcifera.