Abstract: In order to study the effect of non-metallic inclusions on the mechanical properties of ultra-high strength steels for aerospace application, the micro-behavior of inclusion inducing crack initiation and propagation in two ultra-high strength steels under tensile and low cyclic fatigue loading was investigated by means of SEM in situ observations. The influence of inclusions' shape and size on crack initiation and propagation was analyzed. The results show that cracks initiate in a TiN or AIN inclusion alone under tensile loading. The larger the size of an inclusion, the more the number of cracks initiated in the inclusion is, and the lower the initiation stress of the first crack is. Cracks also initiate in a TiN inclusion alone under fatigue loading. But for a chain of AIN inclusions, cracks initiate in the matrix between two inclusions under tensile and fatigue loading. A chain of inclusions is more harmful to the fatigue properties of ultra-high strength steels than an inclusion alone, and an inclusion is more harmful to the fatigue properties than to the tensile properties.