A directed evolution and random mutagenesis were carried out with thermotolerant yeast Kluyveromyces marxianus ATCC 36907 for efficient xylitol production. The final selected strain, K. marxianus 36907-FMEL1, exhibited 120 and 39 % improvements of xylitol concentration and xylitol yield, respectively, as compared to the parental strain, K. marxianus ATCC 36907. According to enzymatic assays for xylose reductase (XR) activities, XR activity from K. marxianus 36907-FMEL1 was around twofold higher than that from the parental strain. Interestingly, the ratios of NADH-linked and NADPH-linked XR activities were highly changed from 1.92 to 1.30when K. marxianus ATCC 36907 and K. marxianus 36907-FMEL1 were compared. As results of KmXYL1 genes sequencing, it was found that cysteine was substituted to tyrosine at position 36 after strain development which might cause enhanced XR activity from K. marxianus 36907-FMEL1.