Abstract: Hydrofracturing in situ stress measurements were used to determine in situ stress states of two drill holes for the pre-feasibility study of an underground reservoir at an engineering site in China.Both the maximum and minimum horizontal principal stresses(S_H and S_h) are by far greater than the vertical stress(S_v) at the same depth,and the SH and Sh are higher than the normal stress state at the depth range,with a relation of S_H > S_h > S_v.The maximum horizontal principal stress direction is nearly(E-W),which is in accord with the general direction of the regional maximum horizontal principal stress determined by measuring points in surrounding areas.The authors suggest that the long axis of the underground reservoir should be oriented in a nearly E-W direction in the future,and the in situ stress state,rock mass structure and rock strength will in the main ensure the rock mass stability.