Oxidation process is considered to be an essential process for a successive carbonization of fibers. Herein, adequate pretreatment is necessary for cellulose fibers with their known characteristics to have weight loss and thermal shrink after oxidation process. In this study, we first pre-treated lyocell based precursor with H3PO4, NaCl and KMnO4 with a variation of time. Thereafter, we finally developed the sample form as activated carbon fibers. With thus-produced samples, we observed surface chemical properties, thermal stability, and thermal pyrolysis. Further, more appropriate method between Friedman and Kissinger was also evaluated by using activation energy as a main constant. With promising results, thermal properties and the value of Tpeak have increased as pretreatment time of lyocell fibers increases. Lastly, Friedman analysis was found to be more preferable than Kissinger analysis with its high regression coefficients.