Abstract: 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) is one of the most toxic environmental pollutants. The goal of this study is to understand the cause of the short lower jaw induced by TCDD in the zebrafish embryos. Embryos are exposed to TCDD (0-1.0 μg/L) at 24 h post fertilization (24 hpf) until the time of observation and studied by gene knockdown, immunohistochemistry staining and in situ hybridization. The results show that TCDD treatment leads to decreases in the size of the lower jaw and reduced Shh gene expression. This effect can be blocked by injection of Morpholino antisense oligos against AhR2, the potential TCDD receptor. We also find Cyclopamine, a Shh inhibitor, caused short lower jaw and decreased cell proliferation in the zebrafish embryos. These results suggest that TCDD might inhibit the expression of Shh through AhR and lead to decreased cell proliferation and short lower jaw in zebrafish embryos.