Precise and accurate fault detection in rectifier diode reduces the time and cost of maintenance and prevents the fault development and more severe faults occurrence. In spite of the importance of open circuit diodes, they are not normally protected in three-phase diode rectifiers for open circuit fault. This is very important in the rectifier of generator systems where voltage control is done based on the output voltage of the rectifier. In this paper, all modes of occurrence of diode open-circuit faults in three-phase rectifiers are categorized and simulated. By using fast Fourier transform analysis, an appropriate feature for fault detection is extracted and k-means algorithm is used to clustering the faults. In order to evaluate the proposed method, the results of the experimental test are classified according to simulation results. In all fault classes, the diagnosis was performed correctly.