The effect of competition by the striped flea beetle (SFB) Phyllotreta striolata (Fabricius) on the feeding and growth of the diamondback moth (DBM) Plutella xylostella (L.）was studied. Low numbers of SFB, or short feeding periods by this insect, will stimulate DBM to feed on the same leaves. Conversely, higher SFB numbers and longer feeding periods inhibited feeding by DBM. DBM carrying capacity varied with host and the level of competition. The carrying capacity of Brassica juncea, B. parachinensis, B. chinensis and R. sativus were lower than that of B. alboglabra and B. oleracea. DBM subject to competition by SFB had significantly different fecundity to those without competitors. Other biological parameters weren’t significantly affected by competition from SFB. Small numbers of SFB stimulated oviposition of DBM but higher numbers inhibited this.