Thermal expansions (TEs) of dense and porous samples were studied for mixed conductor Sr(Co0.9Nb0.1)O3-delta (SCNb). In Ar, the TEs of the porous samples were almost the same as those of a dense sample, which had the character of conventional TEs. In air, on the other hand, the TEs of porous samples were larger than the dense sample. The difference of TE in air resulted from a change in the chemical expansion, which originated from the variation of oxygen vacancy concentration. On decreasing the density, oxygen incorporation from air into SCNb took place at a lower temperature, and the length change upon chemical expansion was increased.