The effect of Al addition on the thermoelectric properties of n-type Fe_0.98CO_0.02Si₂ were investigated for Fe_0.98Co_0.02Si_2-xAl_x (0≤ X ≤0.1) and Fe0.98Co0.02Si2 + ywt%Al (0 ≤ y ≤ 6). Mixture of Fe, Si and Co powders with or without Al were arc-melted in an argon atmosphere to form a button composed of α -Fe₂Si₅ and ε-FeSi phases. The button was ground under -60mesh and Al powder was mixed for Fe0.98Co0.02Si2 powder. Then they were ground (MA) in a conventional ball mill for 360 ks. β-FeSi₂ phase was not formed during MA, but was obtained by hot pressing of MA powders at 1173 K for 1.8 ks under 25MPa. It was found that the thermoelectric properties of Fe_0.98Co_0.02Si₂ was drastically changed by the ddition of Al. The resistivity of Fe_0.98Co_0.02Si₂ increased with increasing Al addition and reached at maximum at x=0.04 and y=1, respectively and thereafter decreased again for higher Al content. Thermoelectric power decreased with increasing Al addition and the property drastically changed from n-type(x ≤ 0.04), (y ≤ 1) to p-type(x ≥ 0.06), (y ≥2). As a result, the power factor(Q²/ρ) of Fe_0.98Co_0.02Si₂ decreased with increasing Al addition. On the other hand, the thermal conductivity of Fe_0.98Co_0.02Si₂ slightly increased with increasing Al addition due to the increase in the heat capacity.