With modern steel manufacture capacity increasing,the demand for techniques of quick and accurate inclusion evaluation of the slab by large scale becomes urgent for the metallurgical technicians,especially the test carried out on the initial steel products.By these test results,the quality of further processed steel product can be forecasted.These techniques involve the investigation on inclusions size,distribution,kind,and quantity.At present,the ultrasonic method is a suitable means to meet this demand.In this paper,inclusions detection sensitivity by focused ultrasonic method on the slab specimen is discussed,and an ultrasonic method by which inclusion as small as 100μm in the slab specimen of 30mm thick can be efficiently tested is proposed by the help of the focused ultrasonic transducer whose working frequency is 7.5MHz.The test result is verified by mechanical dissection and microscope observation at last,the experiment result demonstrates that not only it is feasible to carry out inclusion detection directly on the slab specimen by ultrasonic method,but also the ultrasonic detection sensitivity can be much higher than commonly estimated degree.