Different methods for detecting broken bars in induction motors can be found in literature. Many of these methods are based on evaluating special frequency magnitudes in machine signals spectrums. Current, power, flux, etc are among these signals. Frequencies related to broken rotor fault depend on slip. In industrial environment due to some phenomena - such as load oscillation, other faults and disturbances – obtrusive frequency components appear in the vicinity of fault components; therefore, correct diagnosis of fault depends on accurate determination of motor velocity and slip. The traditional methods typically require several sensors that should be pre-installed in some cases. This paper presents a diagnosing method based on vibration spectrum. Motor velocity oscillation due to broken rotor causes frequency components at twice slip frequency (2sf) difference around speed frequency in vibration spectrum. Speed frequency and its harmonics as well as twice supply frequency, can easily and accurately be found in vibration spectrum, therefore the motor slip can be computed. Now components related to rotor fault can be found. Evaluation of these fault components magnitudes can be a good measure for fault diagnosis.