In this study, the influence of rubber seed kernel (RSK) on Mukah Balingian (MB) coal was investigated. Co-liquefaction was performed in a tubing reactor with tetralin as the solvent at various physical parameters. The study focused on the effects of RSK weight ratio, temperature and reaction time on conversion and products distribution. This paper aims at developing an effective methodology to find the optimum conditions that maximize the co-liquefaction conversion and products by using response surface methodology (RSM). Experiments were planned as per Box-Behnken design experiments. The mathematical model for the co-liquefaction products was developed using RSM with blending the ratio of RSK, temperature, and reaction time as process parameters. The developed RSM model is to determine the optimal process conditions that affect each of the outcomes. The results of this study revealed that the proposed RSM technique managed to find the optimum physical condition, which leads to the maximum value of conversion performance. (C) 2017 Elsevier Ltd. All rights reserved.