Amorphous aluminum phosphate (AlPO4)-supported cobalt catalysts, with and without Ru promotion, were evaluated to study CO hydrogenation activity during Fischer-Tropsch synthesis (FTS). The Ru-promoted catalyst (RuCo/AlPO4) exhibited higher CO conversion and C-8+ selectivity than that of the unpromoted catalyst (Co/AlPO4) and Co/Al2O3 catalyst. Interestingly, filamentous carbon was formed, even at the mild FTS reaction conditions (T = 220-240 degrees C and P = 2.0 MPa), on Co/AlPO4, and it eventually accelerated catalyst deactivation. The presence of an Ru promoter restricted the formation of filamentous carbon and enhanced catalyst stability.