The effect of catalytic reactor bed dilution on product distribution in Fischer-Tropsch synthesis (FTS) over Ru/Co/Al2O3 catalysts was investigated. Ru/Co/Al2O3 catalysts were prepared by a two-step impregnation method and characterized by N-2 physisorption, CO chemisorption, XRD, and TPR techniques. The catalyst bed was prepared by the layered dilution method with four different cases. The dilution of the catalyst bed affected the temperature profile and bed residence time of the catalytic reactor bed. Furthermore, CO conversion and CH4 selectivity were affected by temperature distribution, which varied with the degree of dilution in the fixed-bed reactor. The olefin to paraffin ratio of C-2-C-4 products and the chain growth probability (a) were influenced by the residence time of the catalytic reactor bed, which varied with the layered dilution method. A transition zone was thought to exist between the catalyst layers, which was referred to as an interval. With a long interval, the heat generated by the exothermic reaction over the catalyst layer would not be appropriately transferred from one layer to another, and the readsorption of a-olefin from pellet to pellet would also become more difficult. The dilution of the catalyst layer in the fixed-bed reactor was determined as an important parameter for optimizing product selectivity in the FTS reaction. Based on the results, the pellet to pellet readsorption model of a-olefin was proposed.