Oligotrophic and mesotrophic bogs are common ecosystems in Polissia of Ukraine. After the Chernobyl accident, these ecosystems were contaminated with radionuclides. A common berry plant in these bogs is the cranberry, which is widely used in food industry. The aim of research is to establish dynamics of migration of 137Cs in cranberries during the last 30 years. The obtained results of monitoring studies are necessary for prognosis of radioactive contamination of cranberry. Investigation was conducted on stationary experimental plots (SEP) in Zhytomyr Polissia. 137Cs specific activity was measured on spectrum analyzers with scintillation and semiconductor detectors. Obtained results testify about intensive 137Cs migration on sphagnum bogs of Polissia of Ukraine. Results of investigation conducted after 34 years of Chornobyl accident showed low values of 137Cs specific activity in cranberry (Vaccinium oxycoccus L.) on all stationary experimental plots. This index in 2020 in fresh berries hesitate from 871±82.6 to 286±16.9 Bq/kg (at the permissible level 500 Bq/kg), and in dry shoots – from 1492 (SEP 15) to 6197 Bq/kg (SEP 12). It was found that in the first period (1991-1996) rather significant decreasing of 137Cs specific activity was observed in fresh berries of cranberry – from 3.8 times on SEP 11 to 2.2 times on SEP 12. In the next period (1996-2002) this decreasing was in the limits 1.4-2.9 times that is lower than in previous one. In the following period decreasing of 137Cs specific activity in berries was significantly lower. Revealed regularities of decreasing of 137Cs specific activity in berries during 1991-2020 also confirmed by decreasing of this index in the plant shoots. Total decreasing of this index in dry shoots on the whole period of investigation was: on SEP 11 – from 94590±4236 to 6099±366 Bq/kg (15.5 times), on SEP 12 – from 31800±2325 to 6197±242 Bq/kg (5.1 times), on SEP 15 – from 12120±898 to 1492±112 Bq/kg (8.1 times), on SEP 18 – from 14732±1126 to 1854±112 Bq/kg (7.9 times). Сontinuation of monitoring on stationary experimental plots will allow to predict levels of radioactive contamination of cranberry on the base of mathematical modeling