The seasonal influence of countermeasures for milk after a nuclear accident was investigated using a quantitative decision-aiding technique, cost-benefit analysis. The 15th day of April and July was selected as a typical deposition date of radionuclide for growing season and non-growing season of pastures, respectively. A dynamic food chain model, DYNACON, was used to evaluate the effectiveness of countermeasures for reducing the ingestion dose. The countermeasures considered are (1) a ban on milk consumption and (2) the substitution of clean fodder. The net benefit from the countermeasures was quantitatively evaluated in terms of cost equivalent of avertable doses and monetary costs of implementing the action. It showed a distinct difference for the different deposition seasons of radionuclides. Obviously, the rapid introduction of a countermeasure after a deposition was important in maximizing the cost effectiveness. In most cases, the substitution of clean fodder was more cost effective than a ban on milk consumption.