Abstract: The CO₂ permeability of Yangquan coal matrix-plug (φ 6×13mm) saturated by CH₄ was studied by carrying out CO₂ breakthrough tests at 40℃. The similar tests were performed with inert gas He for comparison. The breakthrough tests were performed in a self-confining flow cell equipped with an online thermal conductivity detector for continually monitoring the signal of CO₂ or He. The imbibition of gas by coal and the influence of the pore gas pressure were investigated. It shows that unlike He there is significant residual CO₂ left in the coal matrix even after CO₂ was flushed by CH₄ for 36h. It reveals that coal has stronger imbibition ability for CO₂, which leads to the difference in its breakthrough time for different CH₄ flushing time. No notable residual He is found during the breakthrough tests. He permeability is significantly higher than that of CO₂. The obtained permeability coefficients of the coal plug with CO₂ or He at the steady state are closely the same for the repeat tests. It implies that no significant structure change involved during the tests. The permeability coefficients varied with the pore gas pressure due to the existence of the influences of the pore wall, the swelling of coal and the adsorption boundary layer.