The results of tests and determination of mechanical properties of unidirectional polymer composite reinforced with T800 carbon fiber under tension and compression in the longitudinal and transversal directions in the temperature range from room to 315°C are presented. The typical type of fracture of specimens under tension in the longitudinal direction and compression in both directions at room temperature and 315°C is shown. It is established that with increasing temperature, all investigated properties of the material decrease, but least of all the elastic modulus and ultimate strength in longitudinal tensile tests. When the glass transition temperature of the epoxy matrix is reached and exceeded, a sharp degradation of such properties of the material in the longitudinal direction as the ultimate strength and the elastic modulus under compression is observed. An almost uniform degradation of the strength and elastic characteristics of the composite in the transversal direction with increasing temperature is noted. The strength and elastic modulus of the polymer composite reinforced with T800 carbon fiber under longitudinal tension have been calculated based on the known values of the ultimate strength and the elastic modulus of the fiber. It is found that the values of the elastic modulus defined during the tests are consistent with the calculated value, while the experimentally obtained value of the ultimate strength of the composite is significantly lower than the calculated one.