Most areas of plain regions in Mazandaran province are planted with rice, and different pesticides and fertilizers are used at high densities to increase the yield of production. Diazinon is the main insecticide that being used to control Chilo supressalis Walker. The main problem is the high groundwater table in the region, and consumption of this water by local people and anthropogenic side effects of water pollutants. This study has focused on the quality of groundwater with an emphasis on water in shallow wells. Water sampling from 10 shallow wells located in seven villages was carried out during summer and autumn of 2006. A one-liter sample was taken from each of the wells in amber glass bottles and was carried to the laboratory. Residues extraction from samples was done using dichloromethane liquid-liquid method, and clean-up was done on thin layer plates. The residues of insecticide in samples were determined by high performance liquid chromatography (HPLC). The used column was in 150×6 mm, and mobile phase was methanol and water with the ratio of 70:30 v:v, at flow rate of 1.250 ml/min. Detection was done using UV detector at 220 nm. The results of study showed that the residues of diazinon in groundwater of Mahmoud Abad area was 0.002 to 0.572 ?gl-1.In some samples the concentration of diazinon residues in water samples was higher than WHO maximum residue limits (0.1?gl-1).