A bucket assembly for replacing an old bucket is provided with a turbine and a method for replacing the same. The bucket to be replaced is removed and then the bucket assembly configured of a blade part, first and second dovetail members, and a coupling member is simply assembled in a male dovetail, without sequentially disassembling the already installed buckets though a notch opening upon replacing the damaged bucket among the already installed buckets in a tangential entry type dovetail, thereby shortening replacement time and replacement costs of the bucket assembly and preventing a secondary damage occurring to the rotor or the rotor wheel during the disassembling process.