Second law analysis of thermal systems is very important because it enables us to determine the irreversibilities and entropy generation occurring in the elements of those systems and it also enables us to determine the exergetic efficiencies of the components of thermal systems. In this experimental study, the air to air heat pump in which R134a is used as refrigerant, that was installed to a room having dimensions of 6000x4000x3800 mm, was tested between the outdoor temperatures of -2.5°C and 4.5°C. The irreversibilities of the compressor, indoor unit, outdoor unit and capillary tube were investigated with respect to the experimental data. Experiments at the same outdoor temperatures were repeated three times under the controlled conditions. Analyses of the results obtained from the experiments were carried out by the computer code that was developed by means of Engineering Equation Solver (EES-V9.723-3D). The irreversibilities of the elements including compressor, indoor and outdoor units and capillary tube increased as the outdoor temperature was increased. The maximum amount of irreversibilities occurred in the compressor while the minimum occurred in the indoor unit.