An important problem at construction of high earth core rockfill dams (ECRD) is danger of hydraulic fracturing of their seepage control element. It is usually considered that the cause of hydraulic fracturing is presence of micro cracks in the core, which open due to deficit of compressive stresses at penetration of reservoir water into it. However, this theory ignores the presence of the force in the soil core which fractures it from the inside, i.e. the pore pressure force. The author proposes the criterion of core crack resistance based on consideration of pore pressure. With the aid of numerical modeling of stress-strain state, the impact of pore pressure on potential development of cracks in the core of a 330 m high ECRD was studied. The considered dam has an inclined sandy loam core. By the results of analysis, it was established that the value and development of pore pressure are greatly influenced by the degree of water saturation S0 of clayey soil at its placement into the dam body. At S0 = 0.9 pore pressure in the core exceeds 4 MPa, and the zone of high pore pressure covers the most part of the core. At that, stresses in the soil skeleton (effective stresses) are still compressive stresses, which formally evidences about the core integrity. However, at S0 = 0.9 the zone of shear strength loss is formed in the core. It may be expected that failure of the core integrity during shear will result in hydraulic fracturing. Thus, pore pressure induces hydraulic fracturing. To prevent the core hydraulic fracturing, it is necessary to reduce pore pressure. Therefore, the clayey soil should be placed with moisture content by 15 % less than the optimal value.