a modulation scheme detecting apparatus includes a first power estimating module , a second power estimating module and a decision module . the first power estimating module is utilized for evaluating a first nominal power of the received signal rotated by a first predetermined phase according to a first modulation scheme . the second power estimating module is utilized for evaluating a second nominal power of the received signal rotated by a second predetermined phase according to a second modulation scheme . the decision module is coupled to the first power estimating module and the second power estimating module , and is utilized for selecting a target modulation from the first and second modulation schemes according to the first nominal power and the second nominal power .