Oil palm empty fruit bunch (OPEFB) was pretreated by NaOH-steam explosion and then fermented to ethanol by separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) processes usingKluyveromyces marxianus G2-16-1 at 40 ºC. The maximum ethanol production by the SHF and SSF processes was 8.09 g/L (22.21 g/L reducing sugar, 0.08 g/g OPEFB) and 13.658 g/L (0.136 g/g OPEFB), respectively, at 48 h. The OPEFB hydrolysate mixed with molasses to 22% (w/v, total sugar) gave an ethanol yield of 61.60 g/L (0.38 g/g total sugar) at 72 h, while molasses alone gave 53.89 g/L (0.34 g/g total sugar). The OPEFB slurry (OPEFBS; OPEFB hydrolysate containing the solid residue of pretreated OPEFB) gave a maximum ethanol yield of 68.77 g/L (0.44 g/g total sugar) when it was mixed with molasses. Scanning electron micrographs of the solid OPEFB residue in the OPEFBS showed yeast cells adsorbed to the OPEFB fibers. The results indicated that ethanol production from molasses mixed with OPEFB hydrolysate was equal to the cumulative sum of ethanol production from each raw material, and the solid OPEFB residue in the OPEFBS increased the ethanol production in the co-fermentation of molasses and OPEFB hydrolysate.