The study aimed to assess the water quality index in order to evaluate the water quality of the irrigation canal network in a full-dyke system situated in Vi Tan commune, Hau Giang province. Eleven physic – chemical parameters, namely, pH, EC, TSS, TDS, DO, COD, BOD5, N-NH4+, P-PO43-, Turbidity and Total Coliform were considered in the analysis by monitoring fifteen sampling locations within 513ha study area for a period of 4 months. Geographic information system is used to represent the spatial distribution of the parameters and the water quality index map was generated. It is apparent from WQI values that the irrigation canal water with WQI values ranging from 68 to 73 (before-building dyke system period) and 10 to 28 (post construction period) were categorized as class II (fairly good) and class IV (grossly polluted), respectively. It was mainly because of high concentration of N-NH4+ due to agricultural activities. The results indicated that the surface water of the areas was highly polluted and it is unsuitable for domestic usage and irrigation purpose unless treated properly.