Abstract: In order to improve the operability of hazard's definition and classification, and to clarify the relationship between the hazard and the hidden danger, the hazard was redefined and classified further based on the existing definition and classification of hazard and then the relationship between hazards and latent danger was explored in detail. The result shows that the hazard should be redefined as the root or state, which could cause harm or loss and are sources of risk,that the hazards should be categorized into root hazards and state hazards,that the root hazards are the root cause of accidents, which are objective and inherent regardless of being identified or not, including tangible and intangible entities including people, things, organizations, and systems,that the state hazards are root danger's unsafe conditons which trigger accidents directly or indireactly, including being in unsafe state and not being in unsafe state,and that the latent danger is the root hazard being in unsafe state.

Key words: hazard, risk, root hazard, state hazard, latent danger, accident