A two-dimensional ultrathin film was defined as being a film thinner than the dimension of an unperturbed Gaussian chain in the three-dimensional state. The segment density of an unperturbed chain in equilibrium in the two-dimensional solid state was investigated using a Monte-Carlo simulation. When the film thickness exceeds 30% of twice the radius of gyration of a three-dimensional unperturbed chain, the segment density is not particularly high in comparison with the three-dimensional state. This means that a polymer chain will behave as a two-dimensional unperturbed chain in the equilibrium state of a two-dimensional ultrathin film, as in a three-dimensional solid state. Therefore, the excess expansion of a polymer chain in a two-dimensional ultrathin polystyrene film with a thickness comparable to twice the radius of gyration of a three-dimensional unperturbed chain might arise from non-equilibrium characteristics. In addition, the relaxation behaviour of a two-dimensional ultrathin film of a styrene-butadiene-styrene (SBS) triblock copolymer was investigated by transmission electron microscopy. The two-dimensional ultrathin SBS film contracted and decomposed into two phases (polystyrene and polybutadiene) from the one phase right after the film preparation. This indicates that the two-dimensional ultrathin film was in a distinct non-equilibrium state right after spreading a solution on the water surface.