Abstract: The silkworm, Bombyx mori, is an important economic insect and a model for Lepidoptera insects；the mutants of silkworm are the valuable resources for physiology, genetics and functional genomics. We discovered a new body shape mutant named Squab (Sq) during the conservation and genetic analysis of the silkworm resources. The heterozygote (Sq/+) could survive, and the length of mutant is just about four-fifths of the normal and is characterized by slight fat abdomen in the middle region and a little narrow thorax; the homozygote (Sq/Sq) is embryonic lethal. The genetic analysis showed that it was controlled by dominant mutant gene. It is confirmed that the mutant gene Sq belongs to the 14th linkage group by crossing it with the marker genes for each linkage group. Three-point test among aojuku translucent (oa), dirty (Di) and Sq showed that the gene locus of Sq is at 34.6 cm on the 14th linkage group in silkworm linkage map. This study has important significance for further research and utilization of this mutant.