The ultrasonic transmission method based on embedded piezoelectric transducers is employed to continuously monitor the cracking process of different strength notched concrete specimens under three-point bending. For comparison, the strain gauge technique is applied simultaneously. An ultrasonic damage index is employed to analyze the onset and propagation of cracking in concretes of three different strength levels. It is found that the damage process could be characterized by three typical stages, i.e., undamaged stage, progressive stage and failure stage, by this damage index. For estimating the onset of cracking, the embedded ultrasonic method is more accurate and sensitive than strain gauge technique. With the increase of strength level of concrete, the difference in initial cracking loads between the results obtained by the embedded ultrasonic method and strain gauge technique is reduced. An obvious decreasing trend in the duration of progressive damage stage with the increase of strength level is confirmed with embedded ultrasonic method.