A new technique has been developed for the determination of ultimate strength in uniaxial extension by testing results of samples of arbitrary shape. This technique consists in the sample failure with spherical indentors and measuring of sizes of rupture surfaces and zones of failured rocks at contacts with indentors for the subsequent estimation of ultimate stress state. The revised formulas were obtained for calculation of ultimate strength in uniaxial extension with using the method of sample failure with spherical indentors, essentially improving the accuracy of the technique.