Inattentional blindness refers to that observers cannot detect some stimuli even if the stimuli stand exactly where their gaze falls. This phenomenon is common among our daily life and contributes to many traffic and medical accidents. We reviewed the papers and found that “limited resource” and “attentional set” take accounts to this phenomenon. In general, the two causes contribute to this phenomenon simultaneously. However, “attentional set” will lead to this phenomenon independent of “limited resource”. More in depth, from the aspect of neuro-mechanism, “limited resource” and “attentional set” acted on the middle stage of brain processing. The unexpected stimuli have ever received some conscious processing in the traditional conscious processing region even when they are not been aware of. The conscious inputs in occipital-parietal areas in the middle stage, the activation level of temporo-parietal junction and intraparietal sulcus determine whether the unexpected stimuli can arrive to conscious level.