RNA interference (RNAi) holds great potential as a new measure for pest management. However, RNAi effectiveness varies greatly in insects from species to species. Insects in the order Lepidoptera are among the most difficult species for utilisation of RNAi-based methods. In this study, we chose the cotton bollworm Helicoverpa armigera as a target for optimising conditions for RNAi-based gene silencing. We used the gene chitinase as the target since its effectiveness has been demonstrated in many other species previously. We found that the target location has little effect on the efficiency of double-stranded RNA (dsRNA)-induced gene silencing as long as it is located within the coding region. The size of the dsRNA must be above 25 bp. The most important factor for higher RNAi efficiency was the dosage of dsRNAs. No silencing effect was observed if dsRNA was less than 0.425 μg/day. A linear increase was observed when dsRNA concentrations increased from 0.425 μg to 10.625 μg/day. Saturation reached at 53.125 μg/day. It was the first report on the optimisation of RNAi conditions in terms of dsRNA target locations, target sizes and dsRNA dosages on the cotton bollworm, and we believe these results should provide a foundation for further research towards utilisation of RNAi-based technologies for the management of this important insect pest.