Abstract：Objective To learn the timeliness and reasons for the delay in discovering outbreaks via a dengue fever outbreak in Zhejiang Province, and try to improve our disease monitoring system. Methods Epidemic and clinic data were collected by searching cases and epidemiological investigation, and clinic behavior of cases in dengue fever outbreak were described, then the risk factors involved in the delay in discovering outbreak were analyzed. Results Totally 46 cases were found in this outbreak, among which 36 (78.28%)cases were infected before.The onset date of the first case we found was on July 15 th, 57 day before the outbreak confirmation. According to the results, 95.65% of cases see a doctor after their sickness, and the rate of three day hospital visiting was 93.36%(42/46), among which the rate of three day hospital visiting before pathogen confirmation was 91.67%(33/36),and there was no statistic significance in diagnostic rate before and after pathogen confirmation(P=1.00). Conclusion Cases in this dengue outbreak presented high and timely outpatient rate. Thus, the delay confirmation of dengue fever outbreak was caused by the low sensitivity of doctors in all levels of hospital. More training on dengue fever diagnosis for doctors should be carried out in the future to control and prevent dengue fever more efficiently.