Three-step continuous process for producing biodiesel from palm fatty acid distillate (PFAD) was optimized by response surface methodology (RSM). PFAD has high content of free fatty acids (FFA) and is not suited for human consumption: normally it is used in soap or in animal feed. The key parts of the three step continuous process took place in helical static mixers (HSMs) used as continuous reactors. The three-step process was optimized by RSM with 5 levels for each of three factors in central composite design (CCD). First step was esterification, then second step was esterification, and third was transesterification. Methyl ester purities of 71.01 wt.% from first step, 95.94 wt.% from second step, and 99.96 wt.% from third step were achieved with total chemical consumption of (115.1 wt.% MeOH, 13.5 wt.%H2SO4, and 5.0 g/L KOH), and total residence time 147 s in the 3 HSMs. In continuous processing the maximum yields were 109.5 wt.% first-esterified oil, 117.0 wt.% second-esterified oil, and 129.0 wt.% crude biodiesel, and purified biodiesel 86.4 wt.%, in the separated phases from first, second, and third steps, and after purification, respectively. Ester purity from three-step process meets the standard specifications for commercial biodiesel in Thailand, US, and Europe. (C) 2018 Elsevier Ltd. All rights reserved.