The toxicity of perilla-chinese Basil, Perilla frutescens whole plant-derived materials to third-instar of larva Plutella xylostella was examined using that of four insecticides and 5 constituents of P. frutescens from other research materials. The active principle of P. frutescens was identified as the sesquiterpenoids α-farnesene by spectroscopic analysis. In leaf-dipping bioassay, α-farnesene (LD50, 36.9) was 3.9 times more toxic than β-farnesene (LD50, 145.2) against P. xylostella larva, based on 48h LD50 values. This compound was less toxic than insecticides. Naturally occurring α-farnesene merit further study as potential diamond back moth control agent.