The charge-transport characteristics of phase-separated blend films of poly(3-hexylthiophene) (P3HT; electron donor) and poly[2,7-(9,9-didodecylfluorene)-alt-5,5-(4',7'-bis(2-thieny1)-2',1',3'-benzothiadiazole)] (PF12TBT; electron acceptor) were visualized by conductive atomic force microscopy (C-AFM). The C-AFM hole-current images clearly showed two phases: an electrically conductive region assigned to the P3HT-rich donor domain and a nonconductive region assigned to the PF12TBT-rich acceptor domain. The hole current in the conductive region was small compared with that of a neat P3HT film with similar thickness, indicating that the P3HT-rich domain contained a large fraction of PF12TBT as a minor component. Thermal annealing initially increased the hole current throughout the P3HT-rich domain because of reorganization of the P3HT chains from their as-cast configurations. Further annealing increased the hole current mainly in the middle of the P3HT-rich domain, but it decreased the hole current in the boundary areas close to the PF12TBT-rich domain owing to the presence of an intermixed region with a gradient of the P3HT/PF12TBT composition ratio. After annealing at temperatures above the glass-transition point of PF12TBT, the widths of the intermixed regions decreased to similar to 30 nm as phase separation proceeded with decomposition of the intermixed region. Such variations in the intermixed region, which were electrically resolved by the C-AFM, accounted for the temperature dependence of the photovoltaic properties of P3HT/PF12TBT blend solar cells.