Abstract: This paper discusses the data of hydraulic fracturing in oil-wells collected from Tangshan,Tianjin and Cangzhou in North China. It has been found that the instantaneous shut-in pressure,measured in wells with sand-filled fissures is definitely higher than that without sand-filled fissures. The authors suggest that such a difference is caused by the supporting action of the sand in the fissures. Therefore,the effect of the supporting action of sand in fissures,must be considered when stress is to be determined by using the data of hydraulic fracturing.The data obtained from oil-wells without sandfilling of fissures are analysed,and then the physical mechanism and the method of explanation for stress measurement by hydraulic fracturing in oil-wells are discussed.Finally,the results of stress measurement are also discussed. The changes of principal stress with depth between 0-4000 meters in these areas are determined. It turns out that the activities of the north-north east faults are of righthanded strike-slip type.