The evolution of the phase separation was investigated for poly(methyl methactylate)(PMMA)/hyperbranched poly(ester-amide)(HBP) blend films on glass substrate by means of phase contrast microscopy. The films with different component ratios show different phase separation processes and phase morphologies. At a film thickness of about several hundreds nanometers, a cylindrical dispersed phase was observed in the films with lower HBP content. The effects of the composition and sample thickness on the formation of the special morphology were also studied. It is found that the interaction between the substrate and HBP and the thickness of blend film are essential factors for the formation of the phase morphology and the appearance of the special cylindrical morphology depends on the component ratio and the film thickness. There is a critical film thickness, above which the special morphology could be observed. The critical thickness varies as the HBP weight percent changes. Our research provides a possible strategic way to obtain polymer films with special structure which are important for an increasing number of applications in wide fields. (c) 2005 Elsevier Ltd. All rights reserved.