The concentrations of permethrin in the Aqua-Reslin® formulation stored in the insecticide mixers/pumps, 16 ultra-low volume (ULV) truck-mounted spray tanks, and a stock container were sampled and analyzed by gas chromatography. The result showed that an average of 55.53% and 70.32% permethrin in Aqua-Reslin were decomposed in the ULV spray truck tanks and in the mixers/pumps, respectively, during the 4-month mosquito off-season. The degradation may directly result in economic loss. Also, the mosquito control efficacy may be compromised if the concentrations are inappropriate. The permethrin analytical method, economic cost, and degradation-prevention methods are discussed in this article.