The interface of phase separated structure of styrene butadiene rubber (SBR) / acrylonitrile butadiene rubber (NBR) prepared by spin-coating from toluene and tetrahydrofuran (THF) solutions has been characterized by Atomic Force Microscopy (AFM) and Differential Scanning Calorimetry (DSC). AFM line profiles of the phase difference images have given the average thickness of the interface of SBR and NBR blends. On the other hand, the volume fraction of the interface has been evaluated from the heat capacity jump at glass transition of each polymer using DSC. By comparing those results, it has been suggested that the interface thickness of SBR/NBR blends depends on the size of phase separated domains and kinds of solvent. These results suggest that the dispersion state of SBR and NBR in each solution influenced the phase separated structure and the interface thickness.