Efficacies of the amounts and methods for chlorinated water to use in disinfection of lettuce were investigated. Concentrations of total and free chlorine in lettuce samples were measured using a colorimetric reaction with diethyl-p-phenylenediamine (DPD). For measurement of the total amount of residual chlorine remaining on samples of lettuce during storage, 100 mg/L DPD samples were used. The residual chlorine content decreased an initial amount of 14 mg/L and further decreased by 42.9% after three minutes, by 92.9% after ten minutes, and diminished to 4 mg/L after seven hours of storage. Measurements made while applying 200 mg/L chlorinated water showed a similar trend. The change in the amount of free available chlorine in the washing water was determined based on storage period and frequency. While washing, the amount of free available chlorine decreased proportionally with time.