Blends of cellulose acetate butyrate (CAB) and polyestercarbonate (PEC) in the composition range of 0-100 wt % in each component were prepared by thermal compounding. The mechanical, thermal, and rheological properties of the polymer blends were studied. Analysis of the DSC curve showed that CAB was an amorphous polymer and plasticized clearly by PEC as long as its addition was less than 40 wt %. On the other hand, PEC was confirmed to be a crystalline polymer and its crystallinity was affected by the introduction of CAB. The finding that CAB and PEC are at least partially miscible was confirmed by the related dynamic mechanical thermal analysis and DSC data. No evidence of transesterification between CAB and PEC was found from the C-13-NMR and GPC data.