This study intends to investigate the individual effects of essential transesterification reaction variables on density and viscosity of produced hazelnut oil methyl ester, and determine reaction variables proving the minimum viscosity. In biodiesel production, catalyst (NaOH) amount, reaction temperature, reaction duration and methyl alcohol (CH3OH) to oil molar ratio were parametrically changed as: 0.25-1.50 %, 40-70 degrees C, 30-120 minutes and 3:1-12:1, respectively. After the parametric investigation, it was determined that 1.00 % NaOH amount, 50 degrees C reaction temperature, 60 minutes reaction duration and 9:1 methanol/oil molar ratio provide the minimum viscosity of 4.128 cSt. Moreover, some other fuel characteristics (flash point, higher heating value, cold filter plugging point and fatty acid composition) of the minimum viscosity biodiesel were specified, and it was reported that these specifications were compatible to EN 14214 and EN ASTM D 6751 standards. (C) 2018 The Authors. Published by Elsevier Ltd.