Brettanomyces custersii CBS 5512 which has reported as a thermotolerant glucose-cellobiose co-fermentable yeast strain was mutated with UV and NTG to improve ethanol yield at higher than 4$0^{circ}C$ B. custersii H1-23, H1-39, H1-55 and H1062 were finally selected for hyper-fermentable strains at higher than 4$0^{circ}C$ from thermotolerant 7510 colonies through 5th selection. Among the selected strains, H1-39 mutant had better fermentability at 4$0^{circ}C$ and 43$^{circ}C$ from different concentrations of glucose. H1-39 and H1-23 mutants yielded more than 70% of the theoretical ethanol yield in 4 and 8% mixed sugars at above 4$0^{circ}C$, which was 5-11% higher than those by original strain. Especially, H1-39 mutant had better fermentability in 4% mixed sugar. It showed 78.5% of the theoretical yield at 4$0^{circ}C$ and 72.2% of the theoretical yield at 43$^{circ}C$. On the other hand, theoretical yield of ethanol by H1-39 mutant in 8% mixed sugar at 4$0^{circ}C$ and 43$^{circ}C$ were 75.2% and 70.2%, respectively. Theses values increased up to 7-11% as compared to those by orginal strain. By the simultaneous saccharification and fermentation, ethanol production by H1-39 mutant increased up to more than 23% as compared to that by original strain.