Interfacial tension gamma of an oligomer blend, oligostyrene (M(w) = 600) and oligo(dimethylsiloxane) (M(w) = 460), was measured by the sessile-drop method. The interfacial tension near the critical point could be fitted to gamma = gamma(0) epsilon(1/T)(mu) where epsilon(1/T) is the reduced temperature distant epsilon(1/T) (=T-c/T - 1) from the critical point T-c. The critical exponent mu was determined to be 1.27, which was exactly of the Ising type, showing an importance of critical fluctuations. The critical amplitude gamma(o) was 5.05 mN m(-1). The critical amplitude of the mean-field type theory was calculated with the interaction parameter chi evaluated from the coexistence curve. The theoretical prediction was reasonably