A concept of crack closure pressure is proposed for evaluating tectonic stresses by the hydraulic fracturing stress measurement technique, based on the linear theory of elasticity and the linear fracture mechanics. The crack closure pressure is defined as a pressure value at a turning point appearing in the decrease rate of the downhole pressure vs the downhole pressure curve after the complete closure of the pressurizing circuit for hydraulic fracturing, i.e., shut-in. This is interpreted as a pressure at the instant when the crack which was induced by the hydraulic fracturing on the surface of a borehole closes completely after the shut-in. The relationship between the crack closure pressure and the tectonic stresses is very simple, and no material constants, such as the fracture toughness and hydraulic properties of the rock, appear in the relationship. Thus, it can be applied as a new and extremely useful formula for evaluating tectonic stresses by the hydraulic fracturing tectonic stress measurement technique.