The paper deals with a significant operational problem encountered on Chernobyl Unit 3 during the last period of operation before December 2000 final shut-down of the plant. The eight main steam relief valves of Unit 3 were replaced in 1999 to the new set of SEBIM valves that caused abnormally high vibration in limits 80-100 % of nominal power and several shutdowns. Power output of the Unit has been strictly limited by 80 % of nominal load. After performing of special investigation the roots of vibration was discovered and corrective actions have been implemented for elimination of Unit 3 operational limits.