The excavation of rock structures in hard rock involves three safety issues: structural failure such as wedge-type failures, brittle failures such as slabbing and spalling, and a combination of these two. Especially at highly stressed levels, the failure process of rock structures are mainly affected and eventually dominated by stress-induced fractures growing preferentially parallel to the excavation boundary. This brittle failure is usually occurred within very small displacement or strain, and thus it is not easy to determine the precursor and initiation stress level of failure in displacement detection method. To overcome this problem, some researchers have tried to adopt the acoustic emission (AE) detection. This study carried out physical model experiments and determined brittle failure initiation stress and failure grade, which are the two important characteristics of brittle failure. Failure initiation stress and failure grade are determined using AE detection as well as visual observation. And the locations of failure were detected in real time by the AE source location. This study showed that AE detection method could be usefully applied to determine the brittle failure initiation.