The aim of the research was to analyze the cost of rainmaking services on maize and cassava in the Tonle Sap River Basin in Sa Kaeo Province, Thailand. The results will assist the Bureau of Royal Rainmaking and Agricultural Aviation to decrease rainmaking service expenditure and increase the effectiveness of the service in the future. Data were collected on costs and returns of maize and cassava grown in the 2008/ 2009 crop year in Watthananakhorn, Klonghad and Ta Phraya districts. Data were collected through interviews with 30 maize and 30 cassava farmers receiving rainmaking services and with the same number of both types of farmers not receiving rainmaking services. Comparisons of maize and cassava farming with and without rainmaking services were then made using regression analysis and ratios of farm cost and rainmaking service cost per unit. The results showed that rainmaking expenditure can be reduced by conducting rainmaking operations only from March to August. The quantity of rainfall affected statistically the maize and cassava yields. The service was less effective at reducing the drought problem in July.