BACKGROUND: The relationship between the particle size of spray‐dried Bacillus thuringiensis NT0423 technical powder and its insecticidal activity against diamondback moth was investigated in laboratory and glasshouse conditions. A variety of sizes of Bt particles were made by homogenisation, followed by pilot‐scale milling to optimise further the current Bt formulation process. RESULTS: In laboratory conditions, homogenised Bt suspension with smaller particles had higher control efficacy against diamondback moth. However, in glasshouse conditions, the smallest particle treatment did not follow the size‐dependent insecticidal activity, rather showing decreased control efficacy. Secondly, air‐jet‐milled Bt powder (16.9 µm) had higher control efficacy than hammer‐milled Bt powder (35.1 µm) in glasshouse conditions. However, double air‐jet milling (5.3 µm) had lower control efficacy compared with one‐time milling. CONCLUSION: These results suggest that NT0423 technical powder with smaller particles is better in controlling diamondback moths, but excessively small particles (<10 µm) possibly reduce the insecticidal activity. Copyright © 2012 Society of Chemical Industry