In this study, the impact levels in environmental impact assessment reports of 6 sewer systems were quantified and discussed. The relationship between the quantified impact levels and the system scale factors of Banghsin, Chungli, Taichung Harbor and Kaohsiung Fonshan regions were constructed and the impact levels of Hualien (HL) and Taichung City (TC) regions were predicted using grey model GM (1, N). Finally, the effects of system scale factors on impact levels were evaluated using grey model GM (1, N) too. According to the predicted results of GM (1, N), the relative errors of topography/geology/soil, hydrology/water quality, air quality, noise, solid waste, terrestrial fauna/flora, aquatic fauna/flora, landscape and traffic in HL region were 19 %, 56 %, 13 %, 29 %, 56 %, 19 %, 13%, 19 % and 16 %, respectively. The relative errors of those environmental items in TC region were 25 %, 33 %, 58 %, 136 %, 33 %, 50 %, 58 %, 50 % and 98 %, respectively. According GM (1, N), plant area (PA) and average flowrate (AF) were the system scale factors that affected the impact levels significantly. So PA and AF were the most significant system scale factors. GM (1, N) was applicable to predict the environmental impact and analyze the reasonableness of the impact. If there is a new sewer system EIA to be reviewed in the future, the official committee of EPA could review the reasonableness of impact levels in EIA reports quickly.