Today petroleum reserves are running low along with the increasing demand of fuel transportation and industries, therefore we are required to find alternative fuels, one of which is derived from vegetable oil, namely Green Diesel. Green diesel is a product mixture of hydrocarbons such as diesel which is produced through a cracking process using catalyst as reaction accelerator. The catalyst is Ni/Al2O3. Aimed to understand the effect of Ni concentration in the palm oil cracking process using Al2O3 catalyst to the product viscosity. The cracking process is carried out using a batch reactor with Ni concentration variations of 5% and 10%, for 10 minutes calculated from the target temperature achieved which is 400°C. The highest amount of product acquired from 10% of Ni which is 20.845 gram from the total of 30 grams raw material, the product of 10% and 5% Ni both has the suitable density 0.8523 g/ml and 0,8187 g/ml which is inside the range of green diesel standard 0.8150-0.8600 g/ml, and while the product of 10% Ni has the most suitable viscosity 4.3876 cSt which is inside the range of green diesel standard 2-4.5 cSt. [ABSTRACT FROM AUTHOR]