Biaxial compression tests with ordinary concrete have been carried out under steady state and transient temperature conditions. The test results show that even small load levels in a second axis alter the mechanical properties of concrete significantly. The stress-strain relationships show a significant dependence on the temperature level and the stress ratio. The strength under biaxial compressive stress is higher than the strength under uniaxial compression. The volumetric strains increase with increasing stress ratios and increasing temperatures. The failure temperature of specimens being biaxially loaded is higher than the one of specimens that are uniaxially loaded. The modes of failure indicate that the tensile deformation is vital for the failure mechanism of concrete.