Abstract: The oblique cracks due to unidirectional tension in brittle materials were analyzed coupling the Sidoroff damage model with fracture mechanics. Boundary equations between damage zone and fracture zone were derived to determine the radial sizes of initial damage and fracture zones due to oblique cracks. A new concept was proposed, i.e. the radial strain energy within the fracture zone near oblique crack tip. Then, a relevant criterion was set for crack propagation, i.e. the criterion for radial minimum strain energy(RMSE) within fracture zone. As a result, the crack orientation at fracture zone boundary and the coordinate of initial cracking point of different oblique cracks were both determined.