Pattern competition and selection of strain localization were observed in rocks under uniaxial compression. The localization may be the trigger mechanism and an effective precursor of eventual rupture in brittle media. It is found that usually two high strain zones form before the eventual rupture. The rapidly increasing strain and strain rate within one of the high strain zones will lead to the eventual rupture of the specimen, and in the meantime, the strain and strain rate in the other high strain zone decrease rapidly. The digital speckle correlation method (DSCM) was used to measure the surface deformation fields.