A study have been done for measuring of Diazinon as a Organophosphorus pesticide in nine Ab-bandan (man-made wetland) in Mazandaran province include Kharjisha, Ramenet, Esmaeelkola, Kordkola, Shrag-e-Larim, Anarmarz, Roshandan, Galeshkola and Langoor from Apr 2010 to Mar 2011. A total of 216 samples were taken from nine Abbandan of the south Caspian lowland. Samples analyzing revealed that Diazinon toxin is observed frequently in aquatic ecosystems throughout the year especially in summer. All Ab-bandans except Kharajisha have shown similar yearly trend. Kharajisha as an aquatic system is fed by Babolroud River has almost triple Diazinon concentration than others in summer. In other seasons, this toxin decreases gradually so that winter and spring have lower amount of it. Due to the vast expansion of agricultural fields in the study area, various contaminants such as Diazinon are leached by rainfall, irrigation and drainage activities and then are conducted to the adjacent rivers and Ab-bandans. Thus, development of irrigation and drainage efficiency and some environmental observations under the small land holding condition could be diminish the negative impacts and consequently maintain the ecosystem balance in this region.