Free volume characteristics in poly(n-alkylphenylsilane)s whose n-alkyl side-group was changed from methyl to n-dodecyl were investigated by positron annihilation technique at room temperature. The average radii of free volume increase in two steps, one is observed with an increase in the length of n-alkyl side-groups from methyl to n-pentyl, the other from n-hexyl to n-dodecyl with an interruption between n-hexyl and n-pentyl. The radius in poly(n-hexylphenylsilane) is smaller than that in poly(n-pentylphenylsilane). This agrees with the result of the theoretical calculation that n-hexyl side chain is entangled leading to suitable packing in the literature.