Abstract: Based on the morphological appearances and descriptions in literature,a simple and convenient method for identifying the sex of diamondback moth(DBM),Plutella xylostella(Linnaeus),was developed.The results showed that the sex of the first-and second-instar larvae was not easily distinguishable by naked eyes or optical microscopic observation.However,the male third-and fourth-instar larva showed a lighter colored fifth abdominal segment than that on the female counterparts.On the male pupae,a lump on each side of a short,longitudinal slip was visible near the gonopore on the 9th abdominal segment.The female pupae had a long,longitudinal slip between the genital pore on the 8th abdominal segment and the oviporus on the 9th segment.The adult males had characteristic brown wings with a wavy edge on the posterior margin and a yorked abdominal end.The female adults,on the other hand,were lighter in color and bigger on body size than the males,and had their abdominal ends appeared round in shape.