Abstract: To define the relationship between the severity of maize dwarf disease and yield loss, experiments were conducted in Lin’an City and Tonglu County, Zhejiang Province, during 2011～2013. The disease incidence, damage and yield losses were assessed on the widely-grown maize varieties Jundan 20 and Jundan 18. Compared with healthy maize plants, infected plants had lower height, shorter internodes and fruiting ears, narrower and shorter leaves, fewer filled grains and lower grain weight. The plants with disease grades 1～4 decreased by 28.8%～81.3% in biomass and by 32.3%～98.9% in grain yield per ear. Yield losses (1.03%~32.13%) were significantly correlated with disease incidence (2.31%~39.44%). The formula for reflection of the relationship between damage and yield loss was developed by regression analysis. The economic threshold was calculated and the index was set for the disease control on the basis of the current conditions and economic benefits of maize production.