Abstract: Mermithidae, as an important natural predator of pests such as Bollworm, has great potential for natural biological control of invasive pests. Unfortunately, the in vitro culture of the nematode has not yet been successful, delaying the commercial application of this pest control method. The key reason for this failure is the inability of the worms to accomplish sex differentiation, sparking a strong interest in this process. Here, we analyzed the differences in gene expression of female and male postparasitic Ovomermis sinensis juveniles by mRNA differential display. In total, 20 gene fragments that had differential expression in male and female worms were isolated, including 8 male- and 12 female-specific ones. Bioinformatics methods were employed to analyze sequences of these fragments, in which ensembl analysis shows 4 fragments have comparable parts with C. elegence’s X chromosome, we speculate those fragments are important genes which influence sex differentiation of Ovomermis sinensis, This data provides an idea for further study of the molecular mechanism of sex differentiation in mermithids.