We collected Xylotrechus rusticus Linnaeus( Coleoptera: Cerambycidae) larvae in the field,and measured six body parameters( head capsule width,pronotum width,antenna spacing,mandible width,body width and body length) for determining larval instar. We analyzed six parameters by frequency distribution,and then tested them by the Crosby growth law and linear regression. The larva of X. rusticus is with 13 instars. Head capsule width,pronotum width,antenna spacing,mandible width,and body length can be used for separating larval instars,except ...