The effect of oil shale semi-coke (SC) on the mineralogy and morphology of the ash deposited on probes situated in the flue path of a circulating fluidized bed (CFB) which burns Zhundong lignite (ZD) was investigated. 10 wt% or 20 wt% SC was added to ZD, which were then combusted in the CFB furnace at 950°C. Two probes with vertical and horizontal orientations were installed in the flue duct to simulate ash deposition. Both windward and leeward ash deposits on probes (P W, P L, P W and P L) were analyzed by using a scanning electron microscopy with energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD), an inductively coupled plasma optical emission spectrometry ICP-OES, and a particle size analyzer. When ZD was burned alone, the P W deposit was comprised of agglomerates (<30 m) enriched in CaSO and Na SiO , incurring significant sintering. The P L and P W deposits, however, were of both discrete and agglomerated particles in similar mineral phases but with coarser sizes. The P L deposit was mainly fine ash particles where Na SiO and Na SO were absent. As SC was added, the agglomerates in both P W and P L decreased. Moreover, SiO and Ca/Na aluminosilicates dominated the mineral phases whereas Na SiO and Na SO disappeared, showing a decrease in deposit stickiness. Likewise, the P W deposit was found less spread on the probe, decreasing its deposition propensity. Na-bearing minerals turned into (Na, K)(Si Al)O and (Ca, Na)(Si, Al) O in the P W deposit. Moreover, Na in the deposits decreased from 32 mg/g to less than 15 mg/g as SC presented. The addition of SC would therefore help alleviate the propensity of ash deposition in the flue path in the CFB combustion of ZD.