High free fatty acid (FFA) reduction in mixed crude palm oil (MCPO) was performed with methanol (MeOH) and sulfuric acid (H2SO4) as acid catalyst using the circulation process through static mixer reactor. In this study, the response surface methodology (RSM) was adopted to optimize the acid value in esterified oil after esterification process (first-step) in lab-scale. The results showed that acid value was reduced from 30 mgKOH g(-1) to 2 mgKOH g(-1), when 19.8 vol.% MeOH, 2.0 vol.% H2SO4, reaction temperature 60 degrees C, 40 L h(-1) of MCPO, 50 min reaction time, and 5-m of static mixer in length, were used in the lab-scale. This recommended condition was used to develop the pilot-scale process in which the scaling up of the FFA reduction from 5 L MCPO of lab-scale to 60 L MCPO of pilot-scale, which was designed on the basis of a simple operation and maintenance. In the pilot-scale process, the lower 1 mgKOH g-1 of acid value was achieved when it was conducted at the reaction time of 50 min. In the base-catalyzed transesterification (second-step) of pilot-scale process, the 98.65 wt.% of methyl ester purity was achieved when the following condition: 20 vol.% MeOH, 8 gKOH L-1 oil, and 60 min reaction time at 60 degrees C, was used to produce biodiesel. (C) 2014 Elsevier Ltd. All rights reserved.