Large amounts of degradable wastes including sewage water and agricultural waste or nutrients that stimulate growth of organic matter are discharged into the Caspian Sea. As a result, average dissolved oxygen concentration in deeper layers is decreasing. This paper presents distribution and seasonal variations of the concentration of dissolved oxygen over the southern shelf of the Caspian Sea adjacent to Iran. The dissolved oxygen data were collected down to 200 m depth in two areas in east (off Babolsar in Mazandaran) and west (off Kiyashahr in Gilan) of the southern coast of the Caspian Sea. Surface dissolved oxygen concentration varied between 7.1 and 10.9 mg/l. Distribution of dissolved oxygen across the depth was in accordance with the temperature structure. The presence of the seasonal thermocline during spring to mid winter significantly affected the concentration of dissolved oxygen across the depth. In autumn, in late October, the dissolved oxygen concentration ranged between 7.6 mg/l below thermocline at 40 m level to less than 5 mg/l at 160 m level and 4.2 mg/l at 200 m level. In winter in late February, in the upper 100 m mixed layer the dissolved oxygen concentration was more than 11 mg/l. The data indicates the possibility of significant decline in dissolved oxygen concentration and serious damage to marine life if algal bloom occurs during the strong seasonal thermocline. The results highlight the necessity of certain measures for an effective decrease in the inputs of degradable wastes and plant nutrients into the Caspian Sea.