In this paper, an optimisation approach is proposed to increase the profit of a commercial hydrocracking unit called Isomax. To represent the system, a full-lump kinetic model incorporating the flow rate of fresh vacuum gas oil (VGO), bed temperatures, recycle flow rate and the catalyst life is developed. This model is capable of predicting the yield of all products, and it improves with respect to the previous works by considering LPG and light gases, fresh VGO and recycle streams as separate lumps. After developing and validating the model, the profit function of the plant, including the value of the products, fresh feed and hydrogen, as well as energy expenses, is optimised by manipulating the bed temperatures, flow rate of fresh VGO and combined feed ratio (CFR) whilst all process limitations and operating constraints are taken into account. During two years of study and considering all mechanical and operational constraints, the results confirm that the decision variables, generated by the optimisation package, can increase the gross profit of the Isomax process to about 8.17%, which is equal to $5.6 million of net profit annually. (c) 2012 Canadian Society for Chemical Engineering