In order to explore the molecular mechanism controlling the expression and regulation of the juvenile hormone diol kinase (JHDK) in Ostrinia furnacalis (Guenée) larvae,sequences of conserved regions of the JHDK gene from different individuals were used to design degenerate primers and a cDNA fragment of 189 bp,encoding 63 amino acids,was amplified from 5^th instar larvae by RT-PCR.This fragment had a predicted molecular weight of 6.78 ku and a pI value of 4.57.Two conserved fingerprint motifs of the GTP-binding proteins ∑3 and ∑1 in the JHDK gene were found.BlastP showed that the amino acid sequence of O.furnacalis JHDK (Of-JHDK) was most similar to that of Manduca sexta JHDK (69%) and its relative similarity to the Bombyx mori and Plutella xylostella JHDK was 55% and 52%.Phylogenetic analysis showed that Of-JHDK was most closely related to B.mori JHDK and least related to P.xylostella JHDK.Semi-quantitative PCR showed that the expression of JHDK in 5^th instar larvae was highest in the midgut.JHDK expression increased in the midgut while decreasing in the hemocytes,fat body and integument tissue as larvae developed.