Abstract: In order to evaluate the water quality in Yongjiang River, Ningbo, the spatio-temporal changes of DO, BOD₅, COD_Cr, TN, TP and Chl-a were studied by analyzing water samples from 13 sections within 12 months. Comprehensive evaluation of water quality in different sections was made in fuzzy mathematics methods. The results showed that DO content was negative correlate with contents of BOD₅, COD_Cr, TN and TP. The main water environmental factors, e.g. BOD₅, COD_Cr, TN, TP and Chl-a, were at lower levels in the upper reaches of the river, at higher levels in middle reaches and at lower levels again at the lower reaches. The seasonal changes of main physical and chemical factors displayed unimodal or bimodal patterns in different sections. Fuzzy mathematics comprehensive analysis revealed that the water qualities of the different sections from the superior to the inferior were described in the following order: section 1>section 2>section 13>section 9>section 11>section 12>section 5>section 10>section 8>section 4>section 7>section 6>section 3. Water quality at the most sections, except for section 1, 2 and 9, met level Ⅴ, and the seasonal change was presented as winter>spring>summer>autumn.