Abstract: Based on the investigation of the criteria of multi-crack propagation in hydraulic fracture of inclined wells, a model depicting the relation between lengths and configurations of different cracks is established. The displacement discontinuity method is applied to study on the of mechanical interaction in hydraulic fracture. Based on the crack propagation criterion, the energy release criterion (G-criterion) is corrected to F criterion that can be used to study crack propagation between cracks. It is shown by calculation of crack interaction factors at inner and outer crack tips that the nearer the distance between cracks, the more serious the interaction. The crack interaction can not only accelerate but also slow down the crack propagation.