The concentrations and profiles of paraquat, a kind of herbicide, were studied in water samples taken from a stream flowing through five villages of Mai Chau province, the Northern Vietnam, during dry and rainy seasons. In this study, paraquat was found at almost the sampling sites and showed an average concentration for paraquat to be 30.69 μg/L and with a maximum of 134.08 μg/L. The herbicide concentration tended to be the highest on the dry season because of the least rainfall and also the highest evaporation rate of water in the stream. For risk assessment of human health, a hazard index (HI) value was calculated for estimating the risk towards the residents. HQ in the dry and rainy season is from 0.0001 to 0.2448 and from 0.0001 to 0.0279, respectively. The results showed a minimum risk; however, there are concerns toward the danger of long-term exposure to the residents from the stream that could affect their life quality.