Abstract: It is significant to formulate effective measures to contain the spread of serious infectious diseases such as COVID-19, especially in the early stage of spread. Effective reproduction number is one of the key parameters to evaluate the effectiveness. It is difficult to estimate the effective reproduction number due to the incomplete symptom onsets. A model based on Monte Carlo method is proposed to estimate the effective reproduction number by using small number of symptom onsets. The control efficacy of Chinese mainland and some international countries is evaluated by the estimation of effective reproduction number. COVID-19 was effectively suppressed in two weeks after introducing A-level control measure at January 20, 2020. The containment of COVID-19 in Republic of Korea has also achieved some progresses.