The present invention relates to a method for monitoring blade frequencies of a wind turbine and a monitoring system for performing the method. By the invention an accelerometer is attached in the center of the rotating hub of a wind turbine and thereby measuring the vibrations of the hub. To extract the vibration signals originating from each blade, the vibration signals are combined with the measuring signals from the azimuth angle sensor (rotating angle), which is normally used for pitch control. Alternatively the azimuth angle is found by using the measured accelerations in two directions representing the centrifugal forces and vibrative forces of the hub in these directions and feeding these signals into a phase locked loop (PLL) unit which establishes a phase of the rotating hub representing the azimuth angle. To further extract each blade frequency a Fast Fourier Transformation (FFT) is used on the signals from the accelerometer. To further monitor a severe and alarming change in each blade frequency each blade frequency is compared to the other blade frequencies and an alarm is set if a given level is reached and the wind turbine is stopped.