This paper presents a new 12-step sensorless drive for brushless dc motor based on back-electromotive force (back-EMF) differences, which are estimated from the disturbance observer (DOB) structure. Availability of rotor position information with a resolution of 30 degrees and commutation without phase shift are the advantages of using the back-EMF differences. Unlike the conventional methods that obtain part of the back-EMF signal from the manipulation of three phase voltages, the proposed DOB structure can access the entire back-EMF profile. By designing a proper low-pass filter in the DOB structure, the back-EMF difference can be well detected with lower sensor noise. Compensation of delay commutation introduced by the filter is also discussed. Experiment results showed that the proposed sensorless drive is capable of running the motor in lower speed, and performs better under different speed and load torque as compared with the conventional method.