We investigated the variations in solution-processed thin films containing pure diketopyrrolopyrrole-containing oligothiophene (SMDPPEH) or SMDPPEH blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) under the different annealing temperatures. The optical properties, microstructures, and surface morphologies of the two types of thin films were characterized with UV-vis absorption spectra, grazing incidence X-ray diffraction (GIXRD), and atomic force microscopy (AFM). SMDPPEH thin film annealed at 100 degrees C exhibited a blue-shift of absorption maximum (lambda(max)) peaks. GIXRD results of two types of films showed that (100) diffraction peak intensities were enhanced by increasing annealing temperatures and their shapes were changed from dot to arc when annealing temperatures were beyond 100 degrees C. AFM results showed that the films grain sizes increased as annealing temperature increased. AFM results also showed obvious large-size phase segregation in the films as annealing temperature increased to 125 degrees C. These results indicated that the crystallinity improved with increasing annealing temperature and a phase transition occurred in SMDPPEH films at the annealing temperature of 100 degrees C. This phase transition may lead to maximum device performance.