Malaria is the most important parasitic disease of people, affecting over 200 million people and causing more than one million deaths each year. Presently, the Southeast Asia including Thailand is still the endemic are for malaria. The surveys of the disease prevalence are useful for prevention and control of disease [Phillips RS. Current status of malaria and potential for control. Clin Microbiol Rev 2001;14:208-26] and have been continuously performed in Thailand. The correlation between rainfall and infection rate of malaria is of interest. Here, the correlation between the rainfall and the prevalence of malaria was investigated. The relation between the rainfall (transformed from the geographical data) and the prevalence of malaria (transformed from the overall infection rate of malaria) is investigated. The least square equation plot rainfall (Y) vs. prevalence (X) is Y=24.82X+987.84 (r=0.76, P<0.01). In conclusion, the prevalence of malarial infection in Thailand may depend on rainfall. Therefore, the surveillance and control of mosquito during the period with high rainfall is recommended. Further, similar study to assess the correlation between the rainfall and prevalence of infection in the other countries is required to fulfill the conclusion.