Abstract: This research has been performed with the aim to find out the optimum condition of operating variables for the densification process of rice straw as an alternative solid fuel. A response surface methodology with three levels (-1, 0 and +1) was used as the experimental design. Independent variables to be optimized include densifying pressure, particle size and moisture content of the raw materials. The experiment was performed using a laboratory scale hydraulic press machine in which a number of 17 treatments were randomly implemented following the Bob-Behnken design. The experimental data on the density, relaxation and durability of solid fuel treated as the response variables were fitted into a quadratic polynomial model. The simultaneous optimization of the response variables has been implemented using Desirability Function (DF) approach, computed with the use of Design Expert software. The results of the research showed that the optimum conditions to produce solid fuels from rice straw biomass were obtained at a pressure of 3002.8 psi, the particle size of 34.90 mesh and raw material water content of 7.77%. The application of the optimum conditions enables to produce a solid fuel having a density of at least 1.0 g cm-3. However, further research is still required to investigate the combustion performance of the produced solid fuels in the real combustor.