For the traditional direct torque control (DTC) system of Brushless DC Motors (BLDCM), the motor was imposed on one fixed voltage vector during a control cycle, which lead to great torque ripples and inconstant switching frequency. In order to suppress the torque ripples and keep the switching frequency constant so that the reliability of the system could be improved, the duty ratio control method was introduced into the DTC system of BLDCM and the effect of four duty ratio determination method for suppressing the torque ripples were studied. By using the duty ratio control method, the acting time of the active voltage vector and zero voltage vector during one control cycle were decided by the duty ratio, so the torque could be controlled more accurately and the torque ripples could be suppressed. The results of the simulation and experiments highlight the validation of the method proposed.