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Pipeline transportation is considered as the most economical and efficient method for transporting large amounts of CO2. The potential for pipeline leakage, possibly resulting in catastrophic accidents, makes it vitally crucial to predict and assess the consequences of accidental CO2 pipeline releases. To investigate the dispersion behavior of high pressure CO2 leaked from a pipeline, a new laboratory scale experimental setup with a total length of 14.85 m and an internal diameter of 15 mm was constructed. Upward releases of supercritical, liquid and gaseous CO2 containing 2%, 4%, 6% mol nitrogen were carried out through an orifice of 1 mm. Evolutions of temperature and movement of dry ice particles in the far-field were studied. An improved dispersion model was proposed to analyze the abnormal low-temperature area found in pure CO2 leakage. Furthermore, the effects of N-2 concentration, initial inner pressure and temperature on the temperature characteristics in the far-field were studied. The results show that the addition of N-2 has different effects on the temperature characteristics for CO2 leakage with different initial phase states. The experimental results can be used to determine safety distances and verify outflow and dispersion models.