The effectiveness of permethrin emulsion spray droplets with (10% v/v) and without mineral oil was evaluated in the laboratory by bioassay using larvae of Plutella xylostella. A microtip nozzle was used to produce spray droplets ranging from 52 - 274 µm in volume median diameter. The 2nd instar larvae were placed on treated leaf discs, and knockdown and mortality were recorded after 1 h and 24 h respectively. In both treatments, as droplet size decreased the dose also decreased without losing the effectiveness of permethrin indicating that small droplets were more efficient than large droplets. When sprayed using droplet size between 70 µm to 100 µm, there was no increase in the effectiveness of permethrin from emulsion spray droplets plus mineral oil compared to emulsion spray droplets alone. However, for droplet sizes greater than 125 µm, the effectiveness was reduced.