Population density of Culex tritaeniorhynchus was annually monitored to predict the possibility of a Japanese encephalitis (JE) outbreak at 10 collection sites throughout Republic of Korea (ROK) during mosquito season from 2011 to 2015. Prevalence of Cx. tritaeniorhynchus in ROK was spatially and timely very variable and was significantly highest at Busan city during August. Monthly average population density of Cx. tritaeniorhychus showed high correlation to the monthly average daily average temperature and monthly average precipitation. Two models for the estimation of occurrence of Cx. tritaeniorhynchus based on annual monthly daily average temperature and monthly precipitation are shown with linear regression equations of exp(0.413×temperature-0.949) and exp(0.01258×precipitation+3.777). JE vector surveillance and vector control is warranted as part of an effective JE management program at ROK.