Metallurgical cokes were subjected to gasification by CO–CO2–N2 gas with blast-furnace-like composition–temperature profile to 1673 K (1400 °C) and annealing under N2 at temperature up to 2273 K (2000 °C). Pore structure of cokes was examined using image analysis. Porosity and pore size were both enlarged under gasification and annealing conditions. The pore structure change during gasification was mainly a result of the Boudouard reaction; the pore structure development upon annealing was attributed to the reactions of mineral matters with carbon and further devolatilization. Annealing and gasification caused a decrease in average pore roundness, an increase in the fraction of low roundness pores and the increase of coalescence points in the pore area. The degradation of coke strength following reaction and annealing was characterized using tensile testing. Both gasification and annealing decreased the mechanical strength of coke. Degradation of more reactive cokes (cokes C and D) by gasification at 1673 K (1400 °C) was visibly stronger in comparison with annealing at the same temperature. Increasing the amount of coalescence points and the degradation of coke microstrength during annealing and gasification were major factors in the degradation of cokes under the simulated blast furnace conditions.