Deactivation of Co-Ru/gamma-Al2O3 Fischer-Tropsch (FT) synthesis catalyst along the catalytic bed over 850 h of time-on-stream (TOS) was investigated. Catalytic bed was divided into four parts and structural changes of the spent catalysts collected from each catalytic bed after FT synthesis were studied using BET, ICP, XRD, TPR, carbon determination, H-2 chemisorption and oxygen titration techniques. Rapid deactivation was observed during first 200 h of FT synthesis. In this case, the deactivation rate was not dependent on the number of the catalyst active sites. It was zero order to CO conversion and independent of the size of active sites. Beyond the TOS of 200 h, the deactivation could be simulated with a power law expression: -dX(CO)/dt = kX(CO)(28.5). The physical properties of the catalyst charged in 1st half of the reactor did not change significantly. Interaction of cobalt with alumina and formation of mixed oxides of the form xCoO center dot gamma Al2O3 and CoAl2O4 was increased along the catalytic bed. Percentage reducibility and dispersion decreased by 2.4-25.5% and 0.5-8.8% for the catalyst in the beds 1 and 4, respectively. Particle diameter increased by 0.8-6.1% for the catalyst in the beds 1 and 4 respectively suggesting higher rate of sintering at last catalytic bed. The amount of coke formation in the 4th catalytic bed was 6 times more than that of in bed 1.