Abstract: 25 years has already elapsed since Daqing oil field was exploited.Since a method of driving oil by hlgh-pressure water injection was applied, the pore pressure of layer is 0.5-1.0MPa higher than the original one in general, thus the fault becomes activated, and this directly influenced the high and stable production of oil in the oil field.#br#To search for the relation between water injetion by high pressure and fault activating, Daqing Petrolium Administrative Bureau has cooperated with Institute of Geophysics.SSB, to observe micro-cracking induced by water injection.#br#In July 1984, the water-injection pressure and the waterinjection quantity were increased in the 10 water-injection wells, which are located in the region of Saertu No.1 fault in Daqing oil field, and at the same time, at a distance of 1 km from the fault, the micro-cracking was observed by borehole seismometer.#br#The result shows that there is a good corresponding relation between the increase of water-injection quantity and the occurence of micro-cracking, and it is confirmed that the direct cause of fault activating is the water-injectlon by high pressure.#br#This is a very important in-situ experiment for induced and controlled earthquakes.