The direct synthesis of isoparaffins from synthesis gas (H-2/CO = 2) was carried out in a consecutive dual reactor system under the conditions of P = 1.0 MPa and W/F = 4.97 g(.)h(.)mol 1. In the upper reactor, Co/SiO2 was used for Fischer-Tropsch (FT) reaction, To primarily decompose the heavy FT hydrocarbons, zeolite beta (4:1 weight ratio) was physically mixed with Co/SiO2. The Pd/beta catalyst with a Pd loading of 0.5 wt % and different Si/Al ratios in zeolites beta was loaded in the lower reactor for hydroconverting the products from the upper reactor. Irrespective of the Si/Al ratio of zeolite beta, the results indicate that the final products showed a sharp carbon number distribution in the range of C-4-C-6 and very high selectivity to branched isomers of C-4-C-6 alkanes. An apparent change of selectivity to isoparaffins was observed with the variation of the Si/Al ratio in Pd/beta catalyst. Moreover, the stability of the catalyst was also influenced by the Si/Al ratios. The effect of the Si/Al ratio on the selectivity of isomers at different times on stream was correlated with the acidity and crystallization of the beta zeolite, which were characterized by ammonia TPD and XRD methods.