The Ha Long Bay, which is the Natural World Herritage, is located in the rapidly developed area. Social - economic activities happened there have caused great pressures on the environment of the Bay. The study is intented to point out issues and trends on water quality and to recommend additional water quality parameters. The study results show that substance concentrations in the water column tend to increase year by year (except the oil content). The substance concentrations during rainy season are higher than those in dry season. There are not so many differences in concentration between surface layer and bottom layer. In general, the water quality in the core zone of the Bay is still good. There are, however, local pollutions on nutrient, inorganic substances, oil and total suspended substance in the buffer zone and the hinterland zone. Therefore, parameters of COD, TOC, PO4, chlorophyll a should be added in the monitoring program.