Surface tensions of oligomer mixtures are theoretically and experimentally investigated in terms of surface enrichment. Surface tension is measured for blends of oligo(styrene) (OS) and oligo(dimetylsiloxane) (ODMS) as a function of composition and temperature in a one-phase region including the vicinity of critical point. With increasing ODMS content, the surface tension decreases dramatically, approaches the value of pure ODMS around 20 wt% ODMS, and then decreases very gradual, exhibiting a little sharp decrease to reach the value of pure component. The temperature dependence of the surface tension of the critical mixture is slightly weaker than those of pure polymer liquids, but shows no anomalous change near the critical point. Composition profiles and surface tension are calculated for the present system by a square-gradient theory, where the system is described as a phase-separated three-component mixture consisting polymer 1, polymer 2, and holes. The experimental results are well described by the theory.