The TlBr‐TlI (thallium bromoiodide) crystal and its fiber preparation method were studied in order to develop a high power CO2laser beam fiber. As a result of this study, the most preferable composition for the fiber was determined to be 42 wt. % TlBr. The fiber (0.5 mm in diameter and 1.5 m in length) at that composition had the following excellent characteristics: (1) a high output power of 138 W (70 kW/cm2), (2) a high total transmissivity of more than 93%/1.5 m, and (3) a bending radius of ultimate 3.5 cm.