This study aims determination of transesterification reaction parameters to produce the lowest kinematic viscosity waste cooking oil biodiesel by using sodium hydroxide (NaOH) as catalyst and ethanol (C2H5OH) as alcohol. For this purpose, the individual effects of main reaction parameters such as catalyst concentration (0.50-1.75%), reaction temperature (60-90 degrees C), reaction time (60-150 min.) and alcohol/oil molar ratio (6:1-15:1) on the kinematic viscosities of produced biodiesels were investigated, respectively. According to results, reaction parameters giving the lowest kinematic viscosity of 4.387 cst were determined as 1.25% catalyst concentration, 70 degrees C reaction temperature, 120 minutes reaction time and 12:1 alcohol/oil molar ratio. (C) 2015 The Authors. Published by Elsevier Ltd.