Substrate loading enhancement in fermenter generates difficulty in mixing that results in low ethanol yield owing to insufficient mass transfer. Thus, this study evaluated the efficiency of a six-blade Rushton turbine and a three-blade marine type propeller in a 3 L stirred tank bioreactor (STBR) by using steam exploded and alkaline pretreated oil palm trunk (OPT) fibers in a unique batch and fed-batch strategy. The fed-batch began with 5% (w/v) solid loading and after 12 h of simultaneous saccharification and fermentation (SSF) reaction, 2.5% (w/v) solid was added in the bioreactor with 6 h interval. The marine and Rushton impellers were compared at 5% (w/v) batch and 20% (w/v) fed-batch SSF, using agitation rate of 100 rpm. At 5% (w/v) solid loading, the highest ethanol concentration and theoretical yield were 15.66 ± 1.71 g/L and 61.69 ± 2.34%, and 12.10 ± 0.98 g/L and 54.01 ± 3.52%, respectively for marine and Rushton impeller. At high solid loading (20% w/v), the marine impeller achieved maximum ethanol concentration of 41.33 ± 2.02 g/L at 72 h of fermentation, with a low power consumption of 54.73 kW. The Rushton impeller resulted in ethanol concentration of 39.08 ± 1.47 g/L at 96 h of fermentation with relatively higher power consumption of 83.62 kW. The marine impeller is therefore efficient with the fed-batch strategy.