Entropic contribution to the interaction parameter ceff in the model incompressible polymer/oligomer system is calculated by the lattice cluster theory (LCT). It is found that in the oligomer solvent, there exists a wide concentration range that the non-combinatorial “entropic interaction” term ceff f1f2 perceptibly counteracts the mean field combinary entropy DSMF. With the increase of the solvent size, both ceff and the ratio ceff f1f2 / DSMF first reach their maximum and finally become trivially to zero. It is worth noting that no any demixing was found in the current calculation. This makes the controversial idea “entropically driven demixing” even elusive. However, we propose that further work on compressible polymer solution with structured monomer will witness the demixing owning to an increased configurational correlation.